Laser excitations often induce structural modifications on solid surfaces via electronic processes, and can be a promising method for creating novel structures that are not achieved by conventional thermal processes. In order to understand the fundamentals of underlying physics and to fully exploit unique features of electronic-excitation effects, we have studied laser-induced surface structural changes of covalently bonded semiconductors and graphite, by means of direct imaging with scanning tunnelling microscopy and probing desorption processes by post-ionization spectroscopy. On covalent semiconductor surfaces, including Si(111)-7x7, -2x1, Si(001)-2x1, GaAs(110)-1x1 and InP(110)-1x1, the excitation-induced modifications are characterized by local bond rupture at intrinsic surface lattice sites, resulting in the formation of surface vacancies and the desorption of constituent atoms from the surfaces. The bond rupture shows several common features. First the rate of bond rupture depends senstively on the type of surface site and atomic species. Second the rate shows a highly suerlinear with respect to the excitation intensity. Third, the resulting morphologies show remarkable Fermi-level effects. We also find different features depending on the basic properties of surfaces; wavelength-dependent rates and resulting morphologies of structural modifications. All the important features can be described reasonably by the mechanism of electronic bond rupture based on two-hole localization. Carbon exhibits various condensed phases composed of sp3- and/or sp2-bond orders, due to a small energy difference between sp2- and sp3-bonded states in carbon materials. Hexagonal graphite forms a crystal structure characterized by planar sp2-bonded network within individual atomic layer and van der Waals bonding between neighboring layers. Unlike the case of semiconductors, femtosecond-laser excitation induces the phase transition of the graphite structure into the structure including sp3-type interlayer bonds. This sp3-bonded phase totally differs from the conventional phases of diamond, but a new crystalline phase that has never been reported. The structural change is not simply governed by excitation intensity but is specifically triggered by p-polarized light, showing the electronic origin of the laser-induced phase transition. The electronic mechanism of the new phase formation is discussed based on the experimental results and theoretical considerations.

In this presentation, I introduce approaches of the first-principles simulation treating electron-ion dynamics within the density functional theory for simulating ultrafast phenomena in condensed matters. My first topic is controlling adsorbed atoms and molecules on graphene sheet with use of femtosecond laser shot. Reduction of graphene oxide and single-side H-desorption from graphane (H-terminated graphene) is demonstrated under irradiation with asymmetric pulse of the femtosecond laser. These calculations suggest non-equilibrium process of graphene-related materials. My second topic is simulating photo-excitation and charge separation in photovoltaic material simultaneously by monitoring electron dynamics under irradiation of light. This calculation can monitor enhancement of dipole moment of polar materials upon photo-excitation. Polar crystal and donor-acceptor pair of organic molecules are shown as examples. Especially the latter example show possible way to design organic materials useful for solar cell. All of above work were done under collaboration with Prof. H. Zhang, Dr. M. Yoon, and Prof. M. Scheffler. The author was supported by MEXT HPCI Strategic Program.

The interaction of femtosecond lasers with materials gives rise to a variety of interesting ultrafast phase transitions. Ultrashort light pulses usually act on time-scales comparable to those characteristic for the motion of ions in solids, and create an extreme nonequilibrium state in which the temperature of the electrons and that of the ionic degrees of freedom differ by many orders of magnitude. As a consequence, ultrafast self-organization processes involving collective (and sometimes also coherent) motion of the ions are initiated. We have studied different aspects of nonthermal melting in InSb. We analyzed the behavior of phonons after laser heating of electrons up to high temperatures. With the help of our calculations it was possible to explain the experimentally found puzzling behavior o the root mean square displacements of ions during the first picosecond after laser excitation [1}. We have recently developed an accurate and fast Molecular Dynamics code based on Density Functional Theory to describe laser-induced ultrafast structural changes in solids. Using this code we simulate the structural response of Silicon to femtosecond laser excitation. For absorbed energies slightly below the melting threshold we find a nonequilibrium phenomenon not reported so far in laser-solid interactions: thermally squeezed phonons. This state is characterized by oscillations of the mean-square amplitudes of atomic displacements at twice the frequency corresponding to the maximum of the phonon density of states. Interestingly, amplitudes below the thermal noise limit are present for short time intervals. This effect is the classical analogue to quantum squeezing of phonons. For absorbed energies corresponding to laser intensities above the melting threshold we demonstrate, by analyzing the time-dependence of the structure factor and the pair-correlation function, that Si undergoes ultrafast nonthermal melting. Moreover, we find that laser induced thermal phonon squeezing constitutes the precursor of nonthermal melting. Finally, we study the influence of excitation of intense femtosecond X-ray pulses on the phonon frequencies of Mg and Cu using the frozen-phonon method. [1] E. S. Zijlstra, J. Walkenhorst and M. E. Garcia, Phys. Rev. Lett. 101, 135701 (2008).

The dynamic behavior of crystalline structures is not only of scientific interest, but has technological importance as well. For instance, laser-induced recrystallization for semiconductor devices, ultra-fast phase transition used in optical recording materials and laser-induced optical switches are studied intensively. To develop advanced materials and processing with better performance, the structure dynamics and transitional mechanisms of these materials in the femtosecond time scale have to be well understood. X-ray or electron diffraction can reveal the crystalline structure, and femtosecond time-resolved diffraction techniques have been developed recently. We have demonstrated that a high repetition rate and low-peak power laser can deliver an amount of X-rays similar to that generated with a low repetition rate and high-peak power laser, which is commonly used for femtosecond X-ray generation. A low-peak power laser is commercially available nowadays, and is much smaller and easy to use compared with a high repetition rate laser. In addition, the new X-ray source can be operated in He ambient because of its lower peak intensity, and this makes the system compact and reliable [1â?"3].
The lattice motion and atom displacement in the unit cell of epitaxially grown VO₂ on c-Al₂O₃ were characterized by this new time-resolved x-ray diffraction (XRD) system. The time-resolved XRD measurements of the Bragg angle, intensity, and width of the diffraction lines simultaneously revealed the phase transition of VO₂ and the atomic motion in the unit cell. The phase transition of the VO₂ from monoclinic to tetragonal phase and the twist motion of vanadium atoms were directly observed in the femtosecond time scale. The unit cell of the low-temperature monoclinic VO₂ transformed into the high-temperature tetragonal phase extremely rapidly (within 25 ps); however, the atoms in the unit cell vibrated for more than 100 ps.
The structural dynamics of the VO₂ crystal will be presented in conjunction with the atomic motion of vanadium during the phase transition, and the mechanism of the photoinduced nonequilibrium process will be discussed.

[1] M. Hada and J. Matsuo, Appl. Phys. B 99, 173 (2010.)
[2] M. Hada, K. Okimura and J. Matsuo, Phys. Rev. B, 82, 153401 (2010)
[3] M. Hada, K. Okimura and J. Matsuo, Appl. Phys. Lett. 99, 051903 (2011)

*This work is partially supported by the New Energy and Industrial Support Organization (NEDO), the (Japanese) Ministry of Economy Trade and Industry (METI) and the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST).

Nano-patterning surfaces with uniform ion bombardment yields a rich phase-space of topographic patterns. Our facility for studies of surface processes at the National Synchrotron Light Source (NSLS) allows in-situ characterization of surface morphology evolution during ion bombardment using Grazing Incidence Small Angle X-Ray Scattering (GISAXS). We use this technique to measure in reciprocal space the kinetics of formation or decay of correlated nanostructures on the surface, effectively measuring the evolution of the height-height correlation function S(q). A linear theory model is used to characterize the early time kinetic evolution during ion bombardment as a function of ion beam incidence angle. We compare the behavior of 1 keV Kr+ irradiated germanium to that of 1 keV Ar+ irradiated silicon, examined in our previous work [1]. [1] C.S. Madi, E. Anzenberg, K.F. Ludwig, Jr., and M.J. Aziz, Physical Review Letters 106, 066101 (2011).

Diarylethenes are photonic organic compounds that undergo a reversible color and structure change upon exposure to visible and UV light. Diarylethenes exhibit thermally irreversible and fatigue-resistant photochromic reactions, and are suggested for potential optical switching and memory applications [1]. Revealing the dynamics of phase transition of diarylethenes immediately after photoirradiation could offer new knowledge of nonequilibrium phenomena in organic materials, and would also be useful and important in applications. To directly observe the phase transition immediately after photoirradiation, both femtosecond-to-picosecond temporal resolution and ångström-order spatial resolution are required. We used a table-top in-air femtosecond X-ray diffraction (FXD) system [2] that has both a few hundred fs temporal resolution and ångström-order spatial resolution. In this study we observed nonequilibrium photo-induced phase transition of a diarylethene. Here we report a time-resolved transmittance change in 1,2-bis(2,4-dimetyl-5-thienyl)perfluorocyclopentene, which is a diarylethene that has a closed-ring (colored blue) and an open-ring (colorless) form. We used the optical pump-probe method for this observation. In addition, we performed static X-ray diffraction measurements (XRD) for the (600) face of a diarylethene single crystal with the FXD system. Two kinds of the diarylethene samples were used in the experiments, thin film (~800 nm thick) for the optical pump-probe experiments and single crystal (~100 m thick) for XRD measurements. In the optical pump-probe experiments, the thin film sample was excited with an ultra-short laser pulse (1 kHz, wavelength  = 800 nm, 130 fs pulse width), and a second harmonic laser ( = 400 nm) probed its transmittance with delay time. The transient transmittance at 400 nm was changed in two stages: one was excitation in less than 500 fs immediately after photo-irradiation, and the second was relaxation of a few ps. This indicated that the closed-ring form was excited electronically to an excited state below 500 fs and relaxed to open-ring form in a few ps. The XRD intensity of the (600) face of the open-ring form of diarylethene was more than ten times stronger than that of the closed-ring form of diarylethene after the phase transition, suggesting that the time-resolved XRD spectrum of diarylethene single crystal can be observed with the FXD system. [1] M. Irie et al., J. Org. Chem. 53, 803 (1988). [2] M. Hada et al., Phys. Rev. B 82, 153401 (2010). *This work is partially supported by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST).

Advances in ultrafast X-ray and electron diffraction have made it possible to probe the structure of matter as it undergoes excitation and phase transitions initiated by ultrafast laser pulses. In semiconductors, the excitation of valence electrons into the antibonding conduction band weakens its covalent bonding, causing expansion due to increased repulsive interactions among atoms. For free electron metals like aluminum, excitation of hot electrons does not appear to affect the lattice stability. The lattice is only affected by electron-phonon collisions. For crystals that are stabilized by the Peierls-Jones mechanism, such as for Bi, femtosecond laser excitation shifts the minimum of the potential energy surface towards a non-Pierls distorted state and causes displacive excitation of coherent phonons of the symmetric A1g mode. Softening of the lattice, induced motion of electrons out of their bonding states, is detected as a downward shift in the frequency of the A1g oscillations. We report on ultrafast electron diffraction studies of the lattice response of Bi nanoparticles to femtosecond laser excitation. By measuring the Bragg angle, a transient lattice compression along the (012) direction is observed over several picoseconds followed by expansion, while along (110) direction only expansion is detected. Our findings show that the current description of the interatomic potential of excited Bi nanoparticles needs to be modified. The anisotropy in the transient decay time after laser excitation, which is observed to be longer for diffraction from (012) lattice planes compared to (110), indicates different directional-dependent mechanism for lattice heating.

Low-temperature epitaxy is important for device fabrication because it can lead to suppressing the introduction of defects such as dislocations and staking faults. The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition was studied. The experiment was conducted in ultrahigh vacuum. An amplified Ti:sapphire laser with ~100 fs pulse width, center wavelength ~800 nm, and operating at 1 kHz repetition rate was split into two beams; one used to ablate a Ge target while the other to excite the substrate during deposition. In situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) were used to study the morphology of the grown QDs. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the morphology of the QDs. For Ge coverage of 12 monolayer (ML), it was observed that the excitation laser reduces the epitaxial growth temperature to 70 °C, a temperature at which no epitaxy is possible without excitation. In another set of experiments, a 40 ns Nd:YAG laser operating at 1064 nm with 10 Hz repetition rate was used for ablation and excitation. Applying the excitation laser to the substrate changed the QD morphology and island density and improved the size uniformity of the QDs deposited at 390 °C. For Ge coverage of 22 ML, the excitation laser reduces the epitaxial growth temperature to 250 °C. Surface diffusion measurement calculated from RHEED recovery curves show that the excitation laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved.

Understanding the dynamics of electronically excited species in solids is essential to forming mechanistic models relevant to controlled materials modification. Irradiation of solid surfaces by UV, or higher energy photons, produces energetic species such as holes and free electrons, that relax to form electron-hole pairs, excitons, and other transient species capable of driving surface and bulk reactions. Photo-stimulated desorption, of atoms or molecules, is often indicative of electronic excited state dynamics of surface states and the material bulk. We use nanosecond lasers to excite specific surface sites, such as terraces and corners, of wide-gap ionic crystals and measure velocities and state distributions of desorbed atoms under highly controlled conditions. Photon energies are chosen to excite specific surface structural features that lead to particular desorption reactions. We have demonstrated that desorbed atom product states can be selected by careful choice of laser wavelength. The photon energy selective approach takes advantage of energetic differences between surface and bulk exciton states and probes surface and bulk dynamics directly. We have previously demonstrated surface-selective excitation of alkali halides. Our recent studies have explored nanostructured metal oxide samples grown by chemical vapor deposition or thin films grown by reactive ballistic deposition (RBD) in addition to cleaved single crystal surfaces. Ultraviolet laser excitation of nanostructured MgO and CaO with is known to result in emission of thermal oxygen atoms when photons with energies (e.g. 7.9 eV) greater than the bulk band gap are used,. On the contrary, when these materials are excited using photons with energies below the band gap (<5.5 eV) desorption of only hyperthermal, i.e. fast, oxygen atoms is observed. The maximum of the corresponding kinetic energy distribution is approximately 0.15 eV in both materials. Recently we have observed a qualitatively new highly hyperthermal (HHT) oxygen atom emission from nanostructured CaO excited in the bulk threshold region by 6.4 eV nanosecond pulses. The kinetic energy distribution of emitted O-atoms peaks at 0.7 eV, which is over four times greater than previously reported. This highly energetic atomic desorption challenges the conventional view that bulk excitation can only induce thermal desorption. Excited state dynamics in solids is inherently complex and greater understanding is gained using a combined experiment/theory approach. Using density functional theory and an embedded cluster method, we propose a mechanism for this process based on the interaction of surface holes with bulk excitons.

Ab initio calculations and experimental data indicate that changes in the force field immediately following irradiation by light and electrons may cause important structural changes in carbon nanostructures. Exposure to specifically shaped femtosecond laser pulses may exfoliate graphite layer-by-layer [1] or convert it to diamond [2]. Photo-activated Stone-Thrower-Wales transformations may modify the morphology at the apex of carbon nanohorns during Raman spectroscopy observations [3]. Irradiation by electrons may significantly improve the structural integrity and mechanical properties of low-quality multi-wall carbon nanotubes grown by Chemical Vapor Deposition [4]. Changes in sound absorption on the sub-nanometer scale, as probed by Damping Force Spectroscopy, can be used to gain information about structural changes in the surface and subsurface region [5]. Since direct observation of atomic-scale processes following specific local perturbations is very hard by experimental means, computer simulations are a welcome alternative to gain insight into the underlying Physics.
[1] Yoshiyuki Miyamoto, Hong Zhang, and David Tomanek, Phys. Rev. Lett. 104, 208302 (2010).
[2] Ramani K. Raman, Yoshie Murooka, Chong-Yu Ruan, Teng Yang, Savas Berber, and David Tomanek, Phys. Rev. Lett. 101, 077401 (2008).
[3] M. Duchamp, R. Meunier, R. Smajda, M. Mionic, A. Magrez, J. W. Seo, L. Forro, B. Song, and D. Tomanek, J. Appl. Phys. 108, 084314 (2010).
[4] T. Fujimori, K. Urita, D. Tomanek, T. Ohba, I. Moriguchi, M. Endo, and K. Kaneko (submitted).
[5] Makoto Ashino, Roland Wiesendanger, Andrei N. Khlobystov, Savas Berber, and David Tomanek, Phys. Rev. Lett. 102, 195503 (2009).

Artificially ordered quantum dot (QD) arrays, where confined carriers can interact via direct exchange coupling, may create unique functionalities such as cluster qubits and spintronic bandgap systems. For quantum dots smaller than 10 nm diameter, with an interdot spacing of 20 nm, the exchange energy between confined electrons on a pair of Ge QDs is about 1 meV (equivalent to 11.6 K). The challenge here is that these dots are smaller than dictated by thermodynamic length scales associated with strain driven self-assembly, and at such small spacings, coarsening should frustrate pattern fidelity and size monodispersity. We employ fine-probe electron-beam irradiation to locally decompose ambient hydrocarbons onto a bare Si (001) surface. These carbonaceous patterns are annealed in UHV, forming ordered arrays of nanoscale SiC precipitates that may template subsequent epitaxial Ge and Si growth to form ordered QD arrays. Using atomic force microscopy and high resolution transmission electron microscopy, we study the structure and epitaxial relationship of Si/SiC and Ge/SiC heterostructures for various growth strategies. We find that Ge deposition at 600°C appears to result in conformal growth and wetting of both the SiC nanodots and the Si regions in between. Annealing at higher temperatures results in significant bimodal broadening of the size distribution. This is believed to be due to non-uniform Si1-xGex accumulation over the carbide nanodots. For optimized process conditions, we have developed â?ocube-on-cubeâ? 3C-SiC nanodots with diameters as small as 6 nm and interdot spacings down to 22.5 nm in 6-dot molecules. Although traditional morphological Ge QDs do not form, we are examining whether the Ge forms electronic quantum dots associated with steep gradients in the strain and composition that lead to confining potentials for electrons. Should this be the case, the system would actually be ideal for the proximal spin interactions of interest here. We do observe effects in our preliminary magnetocapacitance spectra clearly associated with the presence of the quantum dots, but these effects are otherwise still not understood in detail. Support from the DOE Office of Basic Energy Sciences is gratefully acknowledged under grant number: DE-FG02-07ER46421.

We have developed a new process to fabricate arbitrary-shaped, multilevel microstructures in bulk silicon with feature sizes down to 50 nm. This process uses focused high energy proton beam irradiation, followed by electrochemical anodization. We make use of the fact that high-energy protons create significantly more localized defects at their endâ?"of-range than close to the surface. By controlling the fluence of an irradiated area, the resistivity can be controlled and increased locally for selective porous silicon (PSi) formation during subsequent electrochemical anodization. During the electrochemical process, the flow of electrical holes from the back surface bends around the high defect regions to the front surface. As a result PSi forms around these regions, leaving the core region intact.This has enabled us to produce complex free-standing microstructures such as arrays or long wires, grids, wheels, vertically stacked wires and wires which can be controllably bent upwards and downwards in the vertical plane. The two most important factors which determine the wire cross-section dimensions and depth are the irradiation ion fluence and energy. We can controllably vary the width of wires from 1 to 5 µm by varying the fluence of 1 MeV protons and the depth of wires from 2 to 15 µm by varying the proton energy. By using a combination of multiple energy proton irradiation over the range of 200 to 1000 keV, and gray scale masks, different ion penetration depths and multilevel free-standing three-dimensional silicon structures can be obtained. This is the only technique capable of making such complex free-standing structures on bulk silicon after a single step etching. We believe this process is an important development in fields such as silicon photonics, MEMS and Terahertz components. We will also review work on use of ion irradiation combined with thick e-beam pattern masks to produce line and gaps sizes down to 50 nm.

There is an ongoing interest in nanotechnology to decrease dimensions of surface structures below the diffraction limit. Furthermore, parallel large-area structuring of surfaces with light induced processes is of particular interest. Along these lines, we are using nanosphere lithography to create highly ordered triangular gold nanoparticle arrays on fused silica substrates. Afterwards we exploit the localized optical near fields of the gold triangles to overcome locally the ablation threshold of fused silicia. For this purpose, the nanoparticle arrays were irradiated with a single ultrashort laser pulse (35 fs), at a central wavelength of 790 nm. Depending on the polarization and the fluence of the laser light, holes, grooves, or channels, with dimensions well below the diffraction limit have been created. As example, for a polarization along the bisector of the triangular nanoparticles, nanogrooves with a depth of 14 nm, a width at its waist of 45 nm, and a length of 290 nm have been generated, if fluences near the ablation threshold were applied. In contrast, tiny spherical nanoholes with diameters of only 23 nm can be achieved, for fluences significantly below the ablation threshold. The principal shape of the nanostructures is explained by 3D simulations of the energy density of the local fields, using a finite integration technique in time domain. The simulations are performed to small and large triangular gold nanoprisms having side lengths of 70 nm and 320 nm, respectively and thicknesses of 30 nm and 100 nm, respectively. Enhancements of the field at different tips of triangular nanoprisms are demonstrated by changing the polarization. We found an up to 23 times enhanced local field at the tips for the small triangular nanoparticles, which easily explains the ablation, although the laser pulse energy is significantly below the ablation threshold of the fused silica. Finally, we present first studies applying two pulses with different polarisation directions and time delays to generate more complex but predetermined nanostructures on the fused silica in a fast parallel process. With these experiments we get also a glance of the ablation dynamics of the triangular nanoparticle, since the generated structures depend on the ablation process. For example, for two linearly but perpendicularly to each other polarized pulses with a short delay time of approx. 100 fs three holes for each triangle are expected. On the other hand, only two holes are expected if the delay time is longer than several picoseconds. Thus, the crossover from the two to three hole structure yields the ablation time of the nanoparticles. F. Hubenthal, R. Morarescu, L. Englert, L. Haag, T. Baumert, F. Träger, Appl. Phys. Lett., 2009, 95, 063101. R. Morarescu, L. Englert, B. Kolaric, P. Damman, R.A.L. Vallée, T. Baumert, F. Hubenthal, F. Träger, J. Mater. Chem., 2011, 21, 4076.

This paper reports on the experimental and theoretical aspects of the formation of nanostructures on gold and silver films using laser interference patterning (LIP). LIP makes use of the interference of two or more high-power pulsed laser beams to produce periodic structures with a well-defined long-range order in a single-step process with periodicity ranging from a half of the wavelength to tens of micrometers over areas spanning 1 cm2. In this work, laser power densities of 3 W/cm2, 5 W/cm2 and 7 W/cm2 were investigated in a three-beam LIP setup operating at the 355 nm wavelength. The lowest power density resulted in the formation of nanobumps on the film with diameter ranging from 500 nm to 3µm and height ranging from 50 to 200 nm. Application of the mid-level power density resulted in the formation of mushroom structures with diameter ranging from 200 nm to 700 nm and height ranging from 1 to 2 µm. Application of the highest power density allowed a portion of the liquid metal to separate during recoil and pinch-off, and consequently, sharp conical peaks are formed with base diameters in the range 100-300 nm, tip heights in the range 1 µm-2.5µm, and tip diameters around 10 nm. Characterization of the fabricated nanostructures through AFM and SEM reveals the probable mechanisms of their formation through pulsed laser induced melting and resolidification, in combination with surface tension driven effects. A novel finite volume-based inhomogeneous multiphase model has been developed to predict the topography of the fabricated nanostructures. Based on symmetry, a 2.5µm X 2.5µm X 0.2µm domain is selected for the film region, while in order to get a stable pressure boundary condition the surrounding air domain is modeled up to 11µm, along with a 1µm substrate domain. A spatially varying transient power source is applied in the film region. The numerical simulation captures the evolution of the nanostructures on film surface with time, as well as the cooling of the film once the laser is turned off. The inhomogeneous model allows different velocity fields for liquid and solid phases and allows the calculation of air drag forces on the liquid metal. The drag force appears to play a major role in the â?~separationâ?T phenomenon observed at the highest power density. The finite volume model is solved using commercial CFD software (ANSYS CFX), enabling the detailed modeling of the phase change phenomena (melting and re-solidification), the interfacial drag and surface tension forces between molten metal and air, the effect of buoyancy (due to density difference between the different phases) and the conjugate heat transfer between the film and the substrate. The model shows reasonable agreement with the experimentally obtained topographies and provides insights on the flow physics during the formation of the nanostructures. These novel structures have potential applications in field emission devices, surface-enhanced Raman spectroscopy, and cell probes.

Nanocomposites of metallic and/or semiconductor nanoparticles embedded in a polymer matrix exhibit properties that lend themselves to applications such as electronic coatings, optical media, â?osmart windowsâ?, and catalysis. In this work, we describe scalable synthesis processes for production and patterning of polymer matrix nanocomposites (PMNCs) that include femtosecond laser irradiation to target specific functional behaviors for the nanocomposite. A modified, in situ, chemical vapor deposition (CVD), nanoinfusion process was used to nucleate and grow nanoparticles in the bulk of a polytetrafluoroethylene-co-hexafluoropropylene (FEP) polymer matrix. Nancomposites resulting from this type of process are composed of an optically transparent polymer matrix with a random distribution of discrete and stable nanoparticles. Typically, these particles are responsible for the nanocompositeâ?Ts optical properties throughout the visible and near infrared, since FEP does not display absorption in these portions of the spectrum. Metallic nanoparticles synthesized with this process can have a strong optical absorption at their surface plasmon resonance (SPR) frequency and we have utilized this property to selectively irradiate and pattern nanocomposites via femtosecond laser, photothermal heating. When exposed to femtosecond laser pulses, irradiated nanoparticles undergo a rapid rise in temperature leading to a rapid release of heat to the surrounding matrix. If the nanoparticle environment includes species used for chemical vapor deposition, the heat causes a localized decomposition of these precursor species in the immediate vicinity of the nanoparticle, leading to a variety of nanostructures. Using this processing scheme, we have produced nanocomposites that display a 20 nm red shift in the SPR of silver nanoparticles in regions of the material exposed to femtosecond laser pulses. This behavior is in agreement with optical models that predict a red shift due to a core-shell nanoparticle geometry (specifically a 14 nm diameter silver core and a 3 nm WO3 shell) which we confirmed by transmission electron microscopy using imaging and energy dispersive spectroscopy measurements. This process has also been adapted to semiconductor nanocomposite systems such as WO3/CeO2 and WO3/Ag, in order to improve the kinetics and intensity of photochromic effects exhibited by tungsten oxide nanoparticles. These results demonstrate that by using optical masks and laser processing, it is possible to synthesize nanocomposites with a high degree of control over the location, composition, size, and distribution of nanoparticles within a polymer matrix, resulting in patterned materials with tailored electrical, optical, and photocatalytic properties.

Platinum and palladium catalysts are critical for a vast range of chemical and technological applications. However, the growing need to reduce the quantity of these and other precious metal based catalysts has led to major research thrusts aimed at improving catalytic performance via nanostructuring. Despite progress towards directed synthesis of a wide variety of Pt and Pd nanostructures and alloys, little attention has been paid to development of methodologies allowing site-specific integration of these catalysts within micro-scale platforms. Moreover, abilities to dictate catalysis within 3D microenvironments may prove enabling for the development of autonomously powered microfluidics, diagnostics, and sensors as well as energy generation, conversion, and storage devices. We describe a straightforward procedure to integrate arbitrary micropatterns of Pt and Pd within 3D microfluidic environments using multiphoton lithography (MPL). MPL-Pt/Pd materials are comprised of polycrystalline nanoparticles and show excellent electronic, electrochemical, and catalytic properties. We show that peroxide decomposition via direct-write Pt can be used to generate directed gas and fluid flow in three dimensions upon integration within designed 3D structural components. We envision this procedure can be applied broadly for site specific catalysis in microfluidic environments, for instance, toward the design and testing of catalytic micro-pumps and motors.

Two-dimensional nanostructure fabrication using electron-beam (EB) and focused-ion-beam (FIB) has been achieved and applied to make various nanostructure devices. Ten-nm structures are able to be formed by using a commercial available EB or FIB system with 5 â?"10 nm beam diameter and high-resolution resist. In this way, it is considered that the technique of two-dimensional nanostructure fabrication has been established. On the other hand, three-dimensional nanostructure fabrication has been also studied using both EB and FIB induced deposition (CVD). The deposition rate of FIB-CVD is much higher than that of EB-CVD due to factors such as the difference of mass between electron and ion. Furthermore, FIB-CVD has an advantage over EB-CVD in that it is more easily to make a complicated 3-dimensional nanostructures. Because, a smaller penetration-depth of ion compared to electron allows to make a complicated 3-dimensional nanostructures. For example, when we make a coil nanostructure with 100 nm linewidth, electrons with 10-50 keV pass the ring of coil and reach on the substrate because of large electron-range (over a few µm), so it is very difficult to make a coil nanostructure by EB-CVD. On the other hand, as ion range is less than a few ten-nm, ions stop inside the ring. This paper presents a complicated 3-dimensional nanostructure fabrication using FIB-CVD. A carbon-coil with 0.6 µm diameter and 0.08 µm linewidth, which was made by 30 kV Ga+ FIB with carbon containing source (phenanthrene) gas. This demonstrated that FIB-CVD is very useful to make a complicated 3-dimensional structures. Moreover, we report an evaluation of the Young modulus of such amorphous carbon pillars by measuring the resonant frequency of pillars. The spontaneous vibration of pillars was detected in SEM electron beams, and the resonant characteristics were analyzed through the signals of a secondary electron detector.

We report on a Ga+ focused ion beam (FIB) erosion method for creating indium droplet capped InAs semiconductor spike nanostructures and examine their resulting microstructural and electronic properties. Under specific conditions, normal incidence FIB irradiation can erode InAs films to create high aspect ratio spikes, or â?onanospikes,â? with average heights of several hundred nanometers and maximum heights greater than 800 nm. Nanospike formation has been found to proceed via an ion-induced droplet masking process [1]. Metallic In droplets form on the InAs surface due to preferential sputtering of arsenic and nanospikes form under the In droplets as the surrounding material recedes. Nanospike creation using an InAs film grown on an InP substrate has revealed that nanospikes will not form on regions of exposed InP. This difference in the FIB response of InAs and InP may be exploited to create nanospikes in regular arrays by pre-patterning an InAs/InP heterostructure to locally control InAs film thickness and In droplet location. The structures of nanospikes created using both InAs and InAs/InP heterostructures have been examined by transmission electron microscopy (TEM), by scanning transmission electron microscopy (STEM), and by STEM energy dispersive spectrometry (EDS). The nanospikes possess a metallic indium cap, an ion damaged outer layer, and a range of internal structures. Some nanospikes have a single crystalline core that matches the orientation of the original InAs film, while others are heavily ion damaged and polycrystalline. Producing nanospikes by templating as described above has been found to produce a higher proportion of nanospikes with continuous single crystalline cores. Nanospike electrical properties have been characterized using a combined in-situ TEM/nanoprobe technique, which allows for simultaneous TEM imaging and current-voltage measurements. Furthermore, microstructural changes arising from the applied current can be correlated with matching changes in the IV characteristics. The nanospikes have been found to be conductive and show non-Ohmic IV behavior. However, the conductivity of the individual nanospikes varies and is determined by the microstructure of each spike. With this in mind, the nanospike electrical data has been analyzed to determine how nanospike structure controls carrier transport, and nanospikes with crystalline or partially crystalline cores have been found to be the most conductive. The ion-disrupted yet still conductive structure of the nanospikes may make them useful for nanoscale thermoelectric applications. [1] KA Grossklaus and JM Millunchick, â?oFocused ion beam creation and templating of InAs and InAs/InP nanospikes,â? Nanotechnology 22 (2011) 355302.

In nanostructure fabrication is often quite difficult to precisely control and characterize critical dimensions such as the size of a gap between two metallic electrodes. Such a structure is fundamental for the measurements of the electrical properties of single molecules, including small molecules that may become essential components of future devices, and long molecules such as DNA that may lend themselves to extremely rapid sequencing through their electrical signals. Amorphous materials, such as SiO₂ or Pd₈₀Si₂₀, are known to be deformed plastically by MeV ion irradiation. The electronic energy loss of the incoming MeV ions causes an anisotropic deformation consisting of an increase in the sample dimensions perpendicular to the ion beam and a decrease in the dimension parallel to the ion beam. In this work, we fabricate amorphous metallic Pd₈₀Si₂₀ electrodes with a nanometer-sized gap by using MeV oxygen ion irradiation-induced plastic deformation. We also demonstrate that the gap size can be controlled with nanometer precision by in situ feedback control using the field-emission current between the electrodes.

Ubiquitous concerns in device fabrication are nanoscale positioning and the integration of complex combinations of diverse materials, many of which are extremely fragile. Frequently the completed device requires one or more of the constituent materials to be damaged or synthesized under suboptimal conditions, thus compromising the performance of the final structure. We have developed a platform to fabricate multi-component electrode cross-bar structures, where each material can be synthesized under its own ideal conditions. Furthermore, surface treatments and procedures that may otherwise be incompatible can be performed without concern of damage to the other constituent materials. We demonstrate our approach by fabricating an all carbon cross-bar electrode structure comprised of a graphene-graphite heterojunction. Initially, a graphene field effect transistor is fabricated using electron beam and optical lithography. The top graphite electrode is sculpted from a bulk piece of highly oriented pyrolytic graphite with the aid of a focused ion beam (FIB) and integrated nanomanipulator system. This requires real-time shaping, cutting, accurate positioning (circa 100 nm precision) and wiring of the graphite top electrode. Electron transport characteristics of each electrode component and the final heterostructure have been measured. We show that this process is effective for the non-destructive production of micron and submicron-scale multi-layer device structures including other materials such as gold. This fabrication scheme could be extended to produce novel structures such as mechanical resonators, and provide a foundation for combining fragile materials that have otherwise been incompatible with traditional fabrication techniques.

Developing our ability to quickly and accurately assess the presence of potentially harmful biomaterials is an essential endeavor for several reasons, including public health and national security. In particular, current approaches for detecting viruses are quite sensitive, but often require labeling for virus identification and may take hours for detection and up to several days for confirmation. A candidate for rapid, label-free viral sensing was introduced in the 1970s based on a technique in which viruses suspended in an electrolyte were driven through a single pore in a polymer membrane. By characterizing the magnitude and duration of ionic current blockages caused by the virions, their size and geometry could be characterized. However, due to the dimensions of the system (typically microns) relative to the size of viruses (roughly 25 nm to 600 nm), these devices could not achieve the sensitivity and accuracy to compete with other emerging sensing techniques. Recently, however, researchers have begun investigating porting the platform from polymer membranes to silicon-based materials to not only allow for the creation of thinner membranes, but also to enlist a wider variety of fabrication techniques. In this study, we use a focused ion beam to directly mill sub-micron pores in ultra-thin silicon nitride membranes to systematically address the role of the reduced pore diameter and membrane thickness in relation to analyte dimensions. With the ability to vary both the poreâ?Ts length and diameter with nanoscopic resolution, we aim to optimize the performance of this simple system to better address its potential as a successful device. While we find that one pore geometry is insufficient to address the entire range of viral sizes, we also demonstrate the potential of this platform to provide the basis for a highly versatile class of detectors. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Review #532369

Radiation hardness due to three dimensional quantum confinements of carriers in quantum dots (QD) based devices e.g. photodetectors, solar cells etc. , had opened up a new scope of application for them in outer space where devices are exposed to energetic radiations, cosmic rays etc. Here we studied the effect of low energy H^- ion irradiation on single layer InAs/GaAs QDs grown over semi-insulating GaAs substrate by solid source molecular beam epitaxy. Firstly, an intrinsic GaAs buffer layer of 0.5 µm was grown at 590°C. Subsequently the temperature was brought down to 500°C and a thin intrinsic GaAs layer of 0.1 μm was grown followed by the deposition of 2.7 ML of InAs QDs. These QDs were further capped by 0.1 μm intrinsic GaAs layer. The growth rate of GaAs and InAs was kept at 0.72 µm/h and 0.2 ML/s respectively. These InAs/GaAs QDs were further irradiated using H^- ions of energy 50 keV with fluence varying between 5 x 10¹² to 2 x 10¹⁵ ions/cm². Photoluminescence (PL) spectra at 9 K showed an increase in PL emission with H^- fluence up to the value of 2 x 10¹³ ions/cm² and a decrease further, even though it remained higher than the un-irradiated sample up to a fluence of 2 x 10¹⁴ ions/cm². X-ray diffraction pattern depicted that on increasing the fluence from 2 x 10¹³ to 2 x 10¹⁵ ions/cm², the reflection from InAs QDs reduced with an increase in FWHM which indicates that the dot size might have reduced up on irradiation, caused a marginal blue shift of 17 nm in PL emissions. Interestingly, for the sample irradiated with 2 x 10¹³ ions/cm² H^- ions, the ratio of integrated PL intensity increased on increasing the temperature from 9 K and exhibited about ~ 10-fold enhancement at 145 K with almost no shift in the PL peak wavelength; this thermal stability of peak emissions is suitable for optoelectronics applications . Un-irradiated sample exhibited two activation energies of 4 meV (defect related) and 162 meV (thermal escape of carriers). For irradiated samples, the defect with activation energy of 4 meV is completely lost. Activation energies calculated for the sample irradiated with a fluence of 2 x 10¹³ ions/cm² were 23 meV and 336 meV. Increment in activation energy from 162 meV to 336 meV revealed greater quantum confinement. The activation energy of 23 meV is attributed to the ion induced electron traps that contributed captured carriers on thermal activation. These electron traps are displacement defects in GaAs such as As antisites, As vacancies and As vacancy-interstitial complexes, created upon irradiation. We can assume these defects to be responsible for the enhancement of PL efficiency when temperature is elevated from 9 K to 145 K. This is probably the first report of the enhancement in PL efficiency of InAs/GaAs QDs with H^- irradiation . DST, India and MNRE, India are being acknowledged.

Chalcogenide glasses (ChG) provide a medium for many different applications of which radiation detection is a vital area of study. Radiation imparts energy into the ChG causing Radiation Induced Effects (RIE), which span from subtle structural changes, where the molecular structure has a slight shift, to bond breaking and creation of new bonds. These effects generate localized electric fields as well as bending and changing of the molecular structure, which encourage diffusion of fast moving positively charged ions such as Ag⁺ to move throughout the structure and bond with the negatively charged dangling bonds or even the lone pair of electrons of Chalcogen atoms such as Se. The incorporation of the fast ions into the structure directly increases the conductivity of the film therefore the rise in conductivity is ultimately a result of the presence of radiation. The compositional and structural changes in the films over blanket films and devices were studied using Energy Dispersion Spectroscopy (EDS), Raman Spectroscopy and X-ray diffraction (XRD). To observe these following effects, lateral devices were constructed with aluminum electrodes - electrode/nanophase chalcogenide glass/electrode in proximity contact with a source of silver. A bias was applied between the two aluminum electrodes and the conductivity of the film between these electrodes was measured after specified radiation doses. Silver sources were spaced apart from the aluminum electrodes such as not to short the two metals but in the vicinity of the measured film to be a direct supplier of silver ions. These electrodes were placed onto ChG films of different compositions i.e. Ge₂₀Se₈₀, Ge₃₀Se₇₀, and Ge₄₀Se₆₀ which have three distinct structures and react in a distinct manner based on the structure. After fabrication, these devices were radiated with γ rays, performed in the γ cell 200 industrial radiator which is a ⁶⁰Co source which provides a dose rate of approximately 20rad/s. It was observed that each composition reacts differently and performs as threshold switching device once a specific dose range has been achieved. The change in conductivity is as high as five orders of magnitude at the threshold dose proving the concept of silver diffusion and binding with the structure to alter the conductivity. After further radiation some compositions have a tendency to a slight reduction in conduction. This effect was predominantly expressed in measurements of the Ge₂₀Se₈₀ based devices, while the devices based on Ge₄₀Se₆₀ the conductivity increases with dose. At the low intensity of the γ source annealing of the radiation induced effects is not expected to occur. From our compositional specific data about material characterization, the difference in radiation sensitivity has a direct relation to the structure as well as the formation of different diffusion products. The molecular composition of these diffusion products are identified by the XRD studies.

Anodic TiO₂ nanotube arrays possess exciting application potentials in solar cells and photocatalysis. Multiferroic nanocomposite superstructures based on TiO₂ nanotubes will lead to large magnetoelectric effect that can influence several applications including magnetic field sensors, transducers, filters, phase shifters, and memory devices. However, self-organized anodization alone can only produce similar nanotubes across the entire area and lacks the ability to locally modify the nanotubes into heterogeneous open and close tube patterns. Filling and decoration of TiO₂ nanotubes with different foreign materials (conductive metal, oxides, and semiconductor) cannot be locally varied for the existing anodized TiO₂ nanotubes. In this study, highly ordered TiO₂ nanotube arrays with hexagonal arrangement are fabricated by anodizing electropolished and focused ion beam (FIB) patterned titanium surface. More importantly, the anodized nanotubes have been selectively closed by focused ion beams, enabling arranged TiO₂ nanotube patterns of any design. The effect of the initial TiO₂ nanotube diameter on the closing process is studied and the mechanism of the FIB closing of the TiO₂ nanotubes is evaluated. Also, the opening of the closed pores is achieved by using a different, lower voltage ion beam. The mechanism for the closing of TiO₂ nanotubes in selected areas with this lower voltage focused ion beam is also provided. The effect of the ion beam energy and the ion beam current density on the tube opening and closing processes are discussed. These results demonstrate the potential to design more sophisticated nanodevices through the iterative focused ion beam guided TiO₂ nanotube closing and opening process.

Graphene revealed many unique properties, including extremely high carrier mobility and thermal conductivity [1]. Understanding how to controllably modify grapheneâ?Ts properties is essential for its proposed electronic, optoelectronic and thermoelectric applications. We were able to tune the electrical properties of graphene via the electron-beam irradiation. It was observed that single-layer graphene is highly susceptible to the low-energy electron beam irradiation [2]. In this talk, we show that by controlling the irradiation dose one can change, by desired amount, the carrier mobility, increase the resistance at the minimum conduction point, and induce the â?otransport gapâ? in graphene. The change in graphene properties is due to the defect formation in the graphene lattice or on its surface. It is interesting that the changes are reversible by annealing until some critical irradiation dose [3]. The electrical measurements were accompanied by the micro-Raman inspection. The Raman spectra of graphene contains three important features: the disorder D and zone-center G peaks near ~1350 cm-1 and ~1580 cm-1, respectively; and the second-order 2D band at ~2700 cm-1. In pristine graphene samples the disorder D peak is absent suggesting the high quality of crystalline structure. The D peak appears after irradiation and its intensity changes non-monotonically with the irradiation dose. We noted that the modification of graphene properties via irradiation can be monitored and quantified by the changes in the relative disorder D peak intensity, and correlated with its electrical and thermal properties. Our data have important implications for fabrication of graphene devices, which involve scanning electron microscopy (SEM) and electron beam lithography (EBL). The work in Balandin group at UCR was supported, in part, by SRC â?" DARPA through FCRP Functional Engineered Nano Architectonics (FENA) center. The SEM characterization was carried out at UCRâ?Ts Central Facility for Advanced Microscopy and Microanalysis (CFAMM). [1] D. Teweldebrhan and A. A. Balandin, Modification of graphene properties due to electron-beam irradiation, Appl. Phys. Lett., 94, 013101 (2009) [2] G. Liu, D. Teweldebrhan, and A.A. Balandin, Tuning of Graphene Properties via Controlled Exposure to Electron Beams, IEEE Trans. Nanotechnology, 10, 865 (2011)

There have been growing research interests in transparent conductive oxide (TCO) due to passive applications as transparent conductors and recently arising active applications for optoelectronic devices such as transparent thin film transistors and dye-sensitized solar cells (DSCs). Zinc oxide (ZnO) is one of candidates as an alternative TCO for tin-doped indium oxide (ITO) owing to the low cost and abundance of the source materials compared to ITO. The production of highly transparent and conductive ZnO films currently relies on physical deposition methods, such as sputtering, which require expensive and mass energy-consuming facilities. Therefore, there is growing interest in producing inexpensive conductive ZnO films. Aqueous solution syntheses, including hydrothermal growth, electrochemical deposition, and chemical bath deposition, are possible methods of fabricating low-cost ZnO-based TCO films because they enable low manipulating temperatures even below 100°C at ambient pressure and require only much less expensive equipment. However, highly conductive ZnO films are rarely obtained by aqueous solution syntheses because growth heterogeneity of crystal planes generally cause low-density films such as rod arrays and hollow crystals. In this study, we succeeded in preparing transparent conductive ZnO films with a high conductivity by a spin-spray method using trisodium citrate and subsequent UV treatment. In the spin-spray method, a source solution containing metal salt and a reaction solution containing pH adjuster/additives are simultaneously sprayed onto substrates mounted on a temperature-controlled rotating table. The formation of the film proceeds in thin liquid film generated by spraying solutions and table rotation. The results of XRD, TEM showed that the addition of trisodium citrate caused reduction of the facet dependence of the growth rate, and resulted in the densification of the film. The as-deposited films were insulators (> 11 Ω cm) regardless of their density. Therefore, post-deposition UV treatment was examined to introduce shallow donors. The Hall measurement showed drastic decrease in their resistivity, i.e. three orders of magnitude lower (4.4 Ã- 10^-3 Ω cm) with high career concentration (on the order of 10^20 cm^-3) for 1 day UV treatment. Since the resistivity was not recovered but maintained after dark storage for 2 months, it indicated that the phenomenon was not dominantly caused by persistent photoconduction. One possible mechanism is inducing carbon and hydrogen in ZnO lattice as donors by decomposing organic molecules in the films utilizing photocatalytic activity of ZnO itself. Several results supported our assumption; the results of FT-IR showed decomposition of the molecules, however, XPS and TDS showed residual organics in the film. Although the properties of the ZnO films in this study are still insufficient for industrial applications, we believe it opens up low-cost environmental-friendly pas way for TCO.

In the past years strain engineering for decreasing transistors switching time has become an important field in nanoelectronics development. One approach for introducing strain into the n-channel MOS transistor channels is the deposition of high tensile stressed silicon nitride on top of the transistor. Therefore many techniques like in-situ nitrogen plasma treatments and subsequent ultraviolet (UV) curing were developed for increasing stress in as deposited liners. For this UV curing with broadband lamps at and above 220 nm wavelength has become a standard in industrial fabrication. We investigated a new approach of the films response to curing with single wavelengths in the range below 200 and near 220 nm as well as a two-step cure procedure combining the radiation of two different lamps with main peaks at 172 nm and 222 nm in their light emitting spectrum. All experiments were done on PECVD deposited silicon nitride films with a thickness of 63 nm and an initial stress of +450 MPa. After film deposition on 150 mm wafers the samples were broken into pieces of 2x6 cm to guarantee a homogenous radiation over the complete sample. The characterization of stress was done by calculation with Stoneyâ?Ts equation. Therefore a bow measurement was done with an Autofocus system as well as an identification of thickness and refraction index by spectroscopic ellipsometry. A chemical network analysis was done by FTIR transmission absorption spectroscopy in nitrogen atmosphere with subsequent calculation of bonding amounts after Lanford and Rand. All measurements were done before and after curing. The curing itself was done at 450°C in vacuum atmosphere for different times. In our experimental study we will give an overview to changes in stress and chemical bonding for thermally assisted UV curing process with wavelengths of 172 and 222 nm. In respect to our study for curing the samples with a Xe2* lamp (172 nm) we could show that it is possible to eliminate nearly all N-H bonds. Furthermore a small amount of Si-H in our SiXNYHZ films was detected yielding in a small stress increase of +600 MPa because of destruction of Si-N bonds. In contrast for radiation with a KrCl* (222 nm) the samples were cured under the same conditions like at 172 nm wavelength and show a smaller decrease in N-H and Si-H bonding amount. A higher amount of Si-N bonds leads to higher stress increase of +700 MPa. The comparison of these results led to an in-situ two-step curing process with first 172 nm and subsequent 222 nm radiation to get a high loss of hydrogen compounds as well as a higher amount of Si-N bonds. In result of these experiments a stress increase of +900 MPa could be observed, which shows a high potential for application in producing highly stressed tensile films.

Deep subwavelength integration of high-definition plasmonic nanostructures is of key importance for the development of future optical nanocircuitry for high-speed communication, quantum computation, and lab-on-a-chip applications. So far the experimental realization of proposed extended plasmonic networks consisting of multiple functional elements remains challenging, mainly due to the multi-crystallinity of commonly used thermally evaporated gold layers. Resulting structural imperfections in individual circuit elements will drastically reduce the yield of functional integrated nanocircuits. We discuss the use of chemically-grown single-crystalline gold flakes that, after immobilization, serve as an ideal basis for focused-ion beam milling and other top-down nanofabrication techniques on any desired substrate [1]. Using this methodology we obtain high-definition ultrasmooth gold nanostructures with superior optical properties and exploit them for experiments where reproducible geometries and very small gaps are required. As a first example, we discuss polarization control of plasmonic near fields with cross antennas, constituted by two perpendicular dipole antennas sharing a common feed gap [2]. We present extended simulations for symmetric and asymmetric cross structures and demonstrate the possibility to build a localized spot with any arbitrary polarization state in the gap. We show recent results for single-crystalline cross antenna prototypes and present their preliminary characterization by means of two-photon photoluminescence microscopy. As a second example, we exploit the large field enhancement achieved with very small gaps in single-crystalline linear antennas to address the temporal dynamics of multiphoton absorption and luminescence in gold nanostructures. It is now established that two-photon photoluminescence in Au is the result of two sequential single-photon absorption events and that its dynamics is ruled by the relaxation time of the excited hole distribution in the conduction band [3]. Notably, higher order absorption processes have been reported in the literature, however, the underlying physics is not yet well understood. We discuss recent two-pulse correlation measurements of ultrashort laser pulses using the multiphoton photoluminescence of gold dipole antennas. Thanks to the large local field enhancement in the gap, we are able to study the relaxation dynamics for higher-order absorption, namely four-photon processes. Together with a simple model, this study leads to the formulation of a first tentative rationale to describe the various nonlinearities reported in the literature. [1] J.-S. Huang et al., Nature Comm. 1, 150 (2010). [2] P. Biagioni et al., Phys. Rev. Lett. 102, 256801 (2009). [3] P. Biagioni et al., Phys. Rev. B 80, 045411 (2009).

We have observed broad band tunable light emission from the ultraviolet (UV) to the red in metal and non metal implanted Silicon. A low energy (32 keV) Ag and Au metal ion has been implanted in crystalline silicon to achieve plasmonic and non plasmonic enhancement of light emission over a broad spectral range. Confined bound-exciton emission in the UV region can be enhanced at room temperature due to exciton plasmon coupling induced by the Ag induced surface plasmon polaritons where as the ultraviolet region in case of Au implanted can be enhanced by electrostatic image charge effect which is very unique to this system. The Continuous Wave -photoluminescence (CW-PL) along with time resolve PL measurement are used to explain the very short life (nanosecond) of carrier recombination. The recombination life time of the electron-hole pair as estimated from the time resolved PL measurement changes from 2 ns to 400 ps in the presence of Ag ion induced SP polaritons. The recombination life time in the Au implanted in Si gets longer due to the decrease in non-radiative recombination rate where as in the case silver implanted on Si recombination life time gets shorter due to enhancement in the radiative recombination compared to the reference sample (Silicon ions implanted in Silicon). Enhancement at non-resonant plasmon frequencies has been observed due to electrostatic-image charge effects. The emission in the visible (2.4 eV) and UV (~3.3 eV) range can also be significantly enhanced by electrostatic image charge effects induced by Au nanoparticles. The emission in the UV region spectrum shows fine structure splitting at low temperature due to contribution from the confined bound excitons as well as the interface states of Si/SiO2. The recombination of carriers in Si bound exciton is mediated by transverse optical phonon due to the polarization of the surface of bound exciton complex. The low energy side of emission spectrum at low temperature is dominated by 1st and 2nd order phonon replica showing the high quality of the bound exciton complex. Confocal fluorescence imaging provides a nanoscale emission map of the metal ion-embedded crystalline silicon substrate at room temperature. The quantum efficiency of the UV emission from the metal-implanted silicon nanoparticles are comparable to GaN emitters.

Relatively little is known about the dynamics of exciton-plasmon coupling in low-dimensional structures such as ZnO quantum wells and wires. The lifetime of the ZnO band-edge exciton is known to be of order ps, but there are almost no experiments that show how that time scale is affected by proximity to plasmonic metal. Nor is it known how doping of the ZnO affects either the lifetime or the quantum efficiency of these emitters, or how plasmonic interactions could be altered by doping â?" information essential to implementing active plasmonics with ZnO. We have fabricated heterostructures with controllable and well-defined excitonic emission energies and plasmon resonances, including Zn/ZnMgO quantum wells fabricated by pulsed laser epitaxy, and coupled the excitonic emission to metal nanoparticles separated from the quantum wells by a spacer layer whose thickness can be varied. The RMS surface roughness on these heterostructures is less than 3 nm, smooth enough to allow subsequent lithographic fabrication of metal nanoparticle arrays on the heterostructure. The localized surface plasmon resonances (LSPs) of these arrays can be tuned from the near-UV to the near-IR by varying the particle diameter, and by changing from Al to Ag to Au nanoparticles. By varying the barrier width and the MgO thickness in the QWs and bilayer structures respectively, the plasmon exciton coupling dynamics can be mapped as a function of plasmon-exciton spacing. While the large spectral separation between the Al, Ag, and Au SPPs results in a weakly coupled system, tuning the aluminum LSP resonance across the ZnO bandgap will provide a model system for the study of strongly coupled exciton-plasmon systems and give access to such phenomena as vacuum Rabi splitting in the strong-coupling regime with the added dimension of control over emitter-plasmon distance. Furthermore, tuning the Ag and Au LSP resonances across the peak impurity emission will allow for a much clearer understanding of coupling between donor-acceptor pairs and localized surface plasmons. Finally, we note that in addition to the band-edge luminescence, even high-purity ZnO films can exhibit donor-acceptor pair emission in the so-called visible luminescence band that is believed to arise from the superposition of a number of optically active defects. Using femtosecond pump-probe spectroscopy and photoluminescence measurements, we have recently been able to use these heterostructures to provide the first detailed description of the energetics and dynamics of one of these defect states â?"a zinc interstitial defect 2.8 eV above the valence-band edgeâ?" that couples selectively to surface-plasmon polaritons in a rough Ag film.These results also show that it is possible to design plasmonic structures to be sensitive to and to control the radiative dynamics of particular defect states.

The helium ion microscopy (HIM) allows for precise modification of the samples due to the unique character of interactions of the helium ions and the sample material as well as sub-nanometer sized ion probe. Imaging, lithography and nanofabrication possibilities of the HIM provide a very versatile tool for nanometer-sized controlled sculpting. Regarding all these features, one may consider HIM as a tool for samples preparation for transmission electron microscopy. Given the excellent performance of state of the art high resolution electron microscopes the demand for artifact-free sample preparation becomes very high and HIM modification could possibly provide defect-free electron transparent specimens. In this contribution we propose a few HIM-based methods for TEM sample preparation. In particular we report the use of the HIM to make a thin wedge without significant artifacts, the possibility to reshape thin metal lines on an electron transparent membrane and a new method of HIM sample preparation by in-situ heating of the samples during He-beam illumination. We modified platinum bridges with sizes 15*200*300 nm on 100 nm thick SiN membrane into various shapes by HIM. The membrane near the bridge can also be selectively removed. The TEM study shows that Pt lattice is visible up to the edge of the cut, indicating that artifact free cutting of Pt can be done by HIM. Nevertheless comparison of the electrical measurements carried out on HIM-modified and non-HIM-modified samples shows that He ions have some impact on the speed of the grain growth in the sample. The usefulness of HIM to make high quality electron transparent samples at room temperature depends strongly on the material. In case of Si and SrTiO3 the specimen is strongly deteriorated by the HIM manipulation (amorphous areas and even He bubbles are created). In the case of CuxBi2Se3, a topological insulator, defect free thin slice can be made. Conventional ion milling and ultramicrotomy resulted in many artifacts related to surface deterioration. HREM images of CuxBi2Se3 in [001] show the crystalline lattice up to the HIM made edge of the sample, which indicates that HIM can be used for preparation of TEM samples of CuxBi2Se3. The HIM created artifacts are visible as a halo in the HIM image, which is mostly due to the amorphization and/or helium trapping inside this area. Typically this amorphous area extends up to 250 nm from the HIM-removed area depending on the sample. Thus under the HIM conditions used, one cannot make a thin wedge of Si, maintaining its original crystalline structure. However, if the cutting is done at elevated temperature, the HIM generated damage can be prevented. We performed this heating using an in-house made heating holder allowing heating of the sample up to 600 C. TEM investigation of Si samples cut at elevated temperature shows that one can prevent amorphization of the sample and significantly decrease formation of artifacts.

Electron beams serve as powerful sources of both ionization and reduction. While electron beam induced deposition from the gas phase has received considerable attention over the past half century, analogous studies of liquid-phase electron beam induced deposition are limited. The types of materials that may be deposited from the gas phase depend on the availability of gaseous precursors, such as complex metallorganics. Alternatively, most elements are easily dissolved into a liquid solvent, which might subsequently be manipulated by an electron beam. Therefore, deposition from liquid precursors, aqueous and non-aqueous, provides a potential route to greatly expand the variety of materials that may be deposited by such a technique. The approach may also improve the purity and quality of the deposited features. This talk describes electron beam induced deposition of metals, semiconductors, and ceramics from liquid precursor solutions. The technique has been demonstrated to produce features of arbitrary geometry with length scales less than 50 nm and provides great opportunities for nanoscale synthesis.

Subphthalocyanine (SubPc) is a promising material used as an active layer in organic solar cells. An effective process to align the SubPc molecules and increase the mean free path of charge carriers to improve device efficiency is currently unavailable. The use of high electric fields to rotate the B-Cl dipole in SubPc offers a potential means to induce ordering of the molecules. An ultrafast laser irradiation method has been used to induce structure changes in jet printed SubPc films. The Ti:Sapphire 150 fs laser pulses used in this study are focused to a beam diameter of 35μm and are normally incident to the films. It was found that both the laser and a static electric field caused grain coarsening in 50μm thick films. In addition, laser irradiation of 30nm thick Subpc films on Si substrates produced the growth of 6μm by 150nm rods on the surface.

Monodisperse gold clusters have been prepared on surfaces in different charge states through soft landing of mass-selected ions. Gold clusters were synthesized in methanol solution by reduction of a gold precursor with a weak reducing agent in the presence of a diphosphine capping ligand. Electrospray ionization was used to introduce the clusters into the gas-phase and mass-selection was employed to isolate a single ionic cluster species which was delivered to surfaces at well controlled kinetic energies. Using in-situ time of flight secondary ion mass spectrometry (SIMS) it is demonstrated that the cluster retains its 3+ charge state when soft landed onto the surface of a fluorinated self assembled monolayer on gold. In contrast, when deposited onto carboxylic acid terminated and conventional alkyl thiol surfaces on gold the clusters exhibit larger relative abundances of the 2+ and 1+ charge states, respectively. The kinetics of charge reduction on the surface have been investigated using in-situ Fourier Transform Ion Cyclotron Resonance SIMS. It is shown that an extremely slow interfacial charge reduction occurs on the fluorinated monolayer surface while an almost instantaneous neutralization takes place on the surface of the alkyl thiol monolayer. Our results demonstrate that the size and charge state of small gold clusters on surfaces, both of which exert a dramatic influence on their chemical and physical properties, may be tuned through soft landing of mass-selected ions onto selected substrates.

Thin films are widely used in electronic devices like displays, solar cells, integrated circuits or find applications as functional coatings. Recently organic materials like conductive polymers, dyes and highly specific receptors are beginning to replace conventional inorganic films. Many applications require high quality films in terms of homogeneity, cleanliness and crystallinity. Physical vapor deposition in vacuum is the typical method capable to satisfy these claims, but it is restricted to molecules sustaining the high temperature evaporation. Due to their thermal instability, large functional molecules often cannot be used to fabricate thin films with improved properties by thermal evaporation in vacuum. Electrospray ionization (ESI) softly ionizes large and complex molecules without fragmentation and thus represents a gas phase particle source which can be used for vacuum deposition. Recently developed, electrospray ion beam deposition (ES-IBD) has been shown to be a method capable to deposit nonvolatile molecules on surfaces in vacuum.[Phys Chem Chem Phys, 2008, 10, 1079-1090; Small 2006, 2, 540-547; ACS Nano 2009, 3, 2901-2910] Due to the low deposition rate, the morphology and microscopic structure of films grown by molecular ion beams were rarely subject of investigation and so far, neither multilayered nor crystalline growth could be evidenced, due to the low flux of ESI source. Here we present that ES-IBD is a versatile method for the vacuum growth of crystalline organic films, whose constituents are not compatible with conventional, evaporation-based organic molecular beam epitaxy. High flux deposition is demonstrated by taking advantage of cluster ion beams, which increases the material per charge ratio. As a model system we use the organic salts sodium dodecyl sulfate (SDS), a nonvolatile organic salt widely used as surfactant, and sodium citrate (SoCit). The samples are characterized by scanning probe microscopy and x-ray diffraction. SoCit forms 3-dimensional crystalline islands while SDS shows stable, homogenous and multilayered films of inverted membrane configuration. Our results show that ES-IBD allows vacuum growth of non-volatile materials with reasonable rates. Current measurements and ESI mass spectrometry offer full in-situ control over all experimental parameters. These features combined will inspire research with the ultimate goal to establish ES-IBD a large scale vacuum deposition method.

The fabrication of buried silicon oxide films in silicon has been an important technology in semiconductor processing for many decades. The commonly used approach to fabricating these buried silicon oxide films has changed from direct oxygen ion implantation (SIMOX) to other more indirect methods; in one such method, an ion beam is used to precisely define the depth of a silicon cut (e.g. SmartCut) as part of the process used to create a buried layer. The thickness of a typical buried oxide film is on the nanometer scale. No matter how it is made, the buried silicon oxide in silicon is under significant strain. Laser Assisted Chemical Etch (LACE) has been used to excavate and access the buried silicon oxide films. Experimental investigation of excavated buried silicon oxide films provides some new insights. In addition, the combination of ion implantation, LACE, lithographic and/or direct write techniques can be used to advantage in nanofabrication.