A potential commercial surface‐enhanced Raman scattering–active substrate: stability and usability

In this article, a surface‐enhanced Raman scattering (SERS)–active substrate with great stability and usability is reported. It is composited with a two‐dimensional ordered array of Ag spherical caps and an aluminum coating. The ordered aluminum template formed during the synthesis of anodic aluminum oxide film was used as the patterned matrix. After sputtering a Ag layer and then peeling off the aluminum template, the patterned structure was replicated on Ag layer. The aluminum template could serve as a coating that protected Ag from being oxidized. Ultraviolet‐visible reflection measurement was performed to monitor the adsorption process of probing molecules. Taking 4‐mercaptopyridine as probing molecules, SERS spectra were investigated. Comparing with the ordinary Ag film, the patterned Ag layer exhibited better SERS activity. The convenience for preparing, storing, and using makes it a promising candidate for SERS application in the fast field analysis. Copyright © 2012 John Wiley & Sons, Ltd.     

Charge‐transfer‐induced doping at an electron donor (α‐sexithiophene)/acceptor (C60) interface and mobility improvement

We studied various aspects relating to surface charge‐transfer‐induced doping at an organic/organic interface using in situ electrical measurements with a field‐effect transistor (FET) during the formation of the electron donor/acceptor interface. Adsorption of the electron‐accepting molecules (C₆₀) on top of the electron donating molecules (α‐6T) led to an increase in the FET hole mobility in an α‐6T film. Under illumination, the FET hole mobility in the α‐6T film with C₆₀ deposition was significantly increased in comparison with that in the dark due to exciton dissociation at the C₆₀/α‐6T interface, resulting in a large threshold voltage shift. The origin of the mobility increase is explained by the multiple trapping and release (MTR) model in which the mobility is determined by the carrier density. Various phenomena relevant to charge transfer and charge transport at the organic/organic interface are reported and their origins are discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thin PTCDA films on Si(0 0 1): 1. Growth mode

We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.5-2 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a Stranski-Krastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface.     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

Penetration of ammonia molecule into ice film; activation energy analysis using Xe molecular beam irradiation

We have investigated the barrier energy for an ammonia molecule to penetrate into ice film by the use of infrared spectroscopy and Xe supersonic beam. After the ice film on a Pt(1 1 1) surface is exposed to ammonia molecules, an umbrella mode of ammonia molecules adsorbed on the ice film has been observed in infrared spectra. After the irradiation of accelerated Xe beam, we observed an energy shift of the mode of ammonia. The shifted mode is assigned to that of ammonia molecules at the interface between the ice film and the Pt(1 1 1) surface. This indicates that the collision with Xe beam induced the penetration of an ammonia molecule to the interface through the ice film. Using this feature, we estimate a barrier for penetration as 0.28 ± 0.03 eV which is much smaller than the one previously reported for bulk ice.     

Photoemission electron microscopy study of anthracene growth on Si(1 1 1)

The thin film growth of anthracene films on Si(1 1 1) surfaces is studied by photoemission electron microscopy (PEEM). The thin film growth of anthracene on Si(1 1 1) is similar to the growth of pentacene on silicon. Initially a layer of flat lying molecules chemisorbs on the surface. Subsequent growth of fractal islands with standing up molecules proceeds on top of this flat layer.     

Morphology of pentacene films deposited on Cu(1 1 9) vicinal surface

We investigate the morphology of a pentacene (C₂₂H₁₄) film adsorbed on the Cu(1 1 9) vicinal surface by scanning tunnelling microscopy (STM). Thermal treatment of a thick film of molecules generates a long-range ordered structure. Series of molecular rows are alternated with areas where the molecules assume two equivalent orientations. STM data analysis suggests that the ordered structure can be described by a rippled morphology. The behaviour of the film at different annealing temperatures suggests a possible explanation of the film structure as due to an adsorbate-induced modification of the substrate.     

Fabrication of a thin film containing C<Subscript>60</Subscript> derivative nanodomains by photo- polymerization of diacetylene acid

A simple approach for preparing a thin film containing C₆₀ derivative (C₆₀-COOH) nanodomains, based on the mixed Langmuir–Blodgett (LB) film of C₆₀-COOH and diacetylene acid (DA), is reported. From the observations of atomic force microscopy, it is found that many small domains in the mixed LB film are formed, which differ from the flat film of pure C₆₀-COOH. Upon the photopolymerization of DA molecules in the mixed LB film, these nanodomains become much smaller and more homogeneous in size and distribution. PACS 68.37.Ps; 68.18.-g; 64.70.Nd; 68.47.Pe; 68.55.-a     

Multiscale electromagnetic SERS enhancement on self‐assembled micropatterned gold nanoparticle films

In this work, we demonstrate a cascaded, multiplicative electromagnetic enhancement effect in surface‐enhanced Raman scattering (SERS) on periodically micropatterned films made of colloidal gold nanoparticles, prepared by a self‐assembly approach, without implying lithography procedures. The multiplicative enhancement effect is obtained by combining surface plasmon near‐field enhancement due to nanoscale features with far‐field photonic coupling by periodic microscale features. The effect is observed for both internal Raman reporters (molecules attached to the Au colloids before their assembly) and external Raman probes (molecules adsorbed on the samples after film assembly). The ability of the patterned films for far‐field light coupling is supported by reflectivity spectra, which present minima/maxima in the visible spectral range. Finite‐difference time‐domain computer simulations of the electric field distribution also support this interpretation. The fabricated dual‐scale SERS substrates exhibit a good spot‐to‐spot reproducibility and time stability, as proved by the SERS response over a time scale longer than 1 month. The experimental demonstration of this cascaded electromagnetic enhancement effect contributes to a better understanding of SERS and can affect future design of SERS substrates. Moreover, such dual‐scale colloidal films prepared by convective self‐assembly can be of general interest for the broader field of nanoparticle‐based devices. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling of the magnetic shape memory effect in Ni<Subscript>2</Subscript>MnGa films

The magnetic shape memory effect in Ni₂MnGa films is discussed and simulated in terms of the theory of diffuse martensitic transformations. Theoretical temperature and field dependences of the film deformation are obtained. Changes in the film shape under the action of the magnetic field and temperature are calculated.     

Microrelief dependent on the temperature and external field on the free surface of a smectic-<Emphasis Type="Italic">A</Emphasis> liquid crystal

A homeotropically oriented smectic-A film on the surface of a solid substrate with a periodic microrelief is considered. Relationships are derived for describing the dependences of the microrelief-induced deformation of the free surface of the film on the film thickness, the temperature, and the external magnetic (electric) field strength. It is shown that, for a specific choice of the microrelief shape on the substrate surface, a variation in the sample temperature and the external magnetic (electric) field strength makes it possible to control not only the microrelief depth but also the shape of the microrelief formed on the free surface of the smectic-A film.     

Averaged local field intensities in composite films

The photophysical properties of molecules adsorbed in composite films (e.g. surface island films) depend on the local electromagnetic field within the film. The ratio between the average field intensity 〈|E|²〉 in the film and the intensity |E_I|² associated with the incident field is a measure of the electromagnetic contribution to the surface influence on molecular photophysical phenomena. This ratio depends on the film composition and morphology, on the dielectric properties of the pure components making the film and on the frequency, direction and polarization of the incident radiation. Calculations of this ratio as a function of these parameters for several models of composite films are presented. Image interactions and retardation effects as well as radiative damping and finite size contributions to the dielectric response of the films are taken into account. In addition, an estimate of the field inhomogeneity within the film is obtained by calculating also the ratio 〈|E|²〉_shell/|E_I|² associated with the field in thin shells surrounding the dielectric particles which constitute the film.     

New organic FET-like photoactive device, experiments and DFT modeling

We present the possible construction of an organic FET-like photoactive device in which source-drain current through a phthalocyanine ( H₂Pc film is affected by a photo-induced dipolar field in a photoactive “gate” electrode. The influence of the dipolar electric field on charge transfer between H₂Pc molecules is modeled by DFT quantum-chemical calculations on H₂Pc dimers and tetramers.     

Coulomb explosion of nanoclusters that glide along organic surfaces

A model of the Coulomb multiple dissociation (Coulomb explosion) of a nanocluster that glides at an atomic distance along an organic film at a velocity that is lower than the Bohr velocity is proposed. Nanoclusters that consist of identical atoms and films, whose molecules contain substructures of periodic diatomic valence bonds, (for example, CnH_2n ) that have a significant dipole moment are considered. These structures can serve as antennas for IR radiation. It is shown that the dissociation process of a gliding nanocluster is induced by a picosecond pulse of highly intense photons (10¹⁴–10¹⁶ W/cm²) that are radiated by IR antennas of the film as a result of the relaxation of collective vibrating excitations that are accumulated in the antenna (excimoles). These excimoles resonantly exit from the IR antennas of the film as a result of the action of the periodic Coulomb field, which appears during the gliding of a nanocluster with respect to the film molecules at a rate below the Bohr velocity. In the framework of the proposed model, the experimental results on the decay of C ₆₀ ⁺ ions as they glide along an organic film that contains molecules with IR antennas are analyzed.     

Influence of surface temperature and surface modifications on the initial layer growth of para-hexaphenyl on mica (0 0 1)

Ultra-thin films of para-hexaphenyl (6P) were prepared on muscovite mica (0 0 1) utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The 6P growth characteristics were studied as a function of substrate temperature and substrate surface conditions. For the initial state of layer growth, thermal desorption spectroscopy (TDS) was used to verify the existence of a wetting layer. In this monomolecular continuous wetting layer, the molecules lie flat on the surface and are rather strongly bonded. For thicker films, in-situ X-ray photoelectron spectroscopy (XPS) in combination with (TDS) was applied to reveal the kinetics of the layer growth. Ex-situ atomic-force microscopy (AFM) was used to determine the film morphology. In particular, the influence of surface modifications (carbon contamination, sputtering) on 6P layer growth was investigated. XPS and low energy electron diffraction (LEED) were used to characterize the mica surface before the film deposition. TDS and AFM revealed a considerable change in film growth, from a needle-like island growth of flat laying molecules on top of the wetting layer (for the air cleaved mica) to terrace-like film growth of standing molecules, without a wetting layer (after surface modifications).     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Controlling molecular orientation of OMBE grown 6P thin films on mica(0 0 1)

The growth of para-sexiphenyl thin films by organic molecular beam epitaxy (OMBE) on mica(0 0 1) was investigated. The morphology of the films was qualitatively and quantitatively analyzed by atomic-force microscopy. Synchrotron radiation was used in order to extract information about orientation of the individual molecules with respect to the substrate. Controlling the growth environment inside the growth chamber allowed to reproduce one-dimensional film morphologies with partially oriented crystallite chains usually obtained by hot wall epitaxy (HWE). Following a dedicated pregrowth procedure results in terraces of upright standing molecules so far not obtained. Phase imaging was used to clearly distinguish film and substrate.     

Irradiation of amorphous Ta<Subscript>42</Subscript>Si<Subscript>13</Subscript>N<Subscript>45</Subscript> film with a femtosecond laser pulse

Films of 260 nm thickness, with atomic composition Ta₄₂Si₁₃N₄₅, on 4″ silicon wafers, have been irradiated in air with single laser pulses of 200 femtoseconds duration and 800 nm wave length. As sputter-deposited, the films are structurally amorphous. A laterally truncated Gaussian beam with a near-uniform fluence of ∼0.6 J/cm² incident normally on such a film ablates 23 nm of the film. Cross-sectional transmission electron micrographs show that the surface of the remaining film is smooth and flat on a long-range scale, but contains densely distributed sharp nanoprotrusions that sometimes surpass the height of the original surface. Dark field micrographs of the remaining material show no nanograins. Neither does glancing angle X-ray diffraction with a beam illuminating many diffraction spots. By all evidence, the remaining film remains amorphous after the pulsed femtosecond irradiation.     

Magnetotransport properties of nano-constriction array in La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> film

Nano-constriction array in La_0.67Sr_0.33MnO₃ film was fabricated by using ion beam etching masked by a monolayer of packed and ordered array of SiO₂ microspheres. Nano-constrictions of around 50 nm in width were fabricated. The low field magnetoresistance (LFMR) exhibited in the samples were observed to be current dependent and the I-V characteristics of the film were found to be nonlinear. These observations were attributed to the co-existence of the ferromagnetic regions and the nano-constricted region of weakened ferromagnetic coupling where Mn³⁺-O-Mn⁴⁺ bond were distorted due to the ion beam bombardment. The spin polarized bias current would strengthen local ferromagnetic coupling when passing through this nano-constricted regions. This current effect is relatively large comparing to the external magnetic field to the drop of resistance.     

Wavelength-tunable photoconductivity of dye-sensitized TiO₂ nanoparticle films

We report in this Letter that wavelength-tunable photodetectors (PDs) can be fabricated by dye-sensitized TiO₂nanoparticle film. The photoelectric response of the detectors is fast. The photocurrent intensity strongly depends on the absorption wavelength of the dye; thus the on/off ratio as a function of light wavelength can be tuned by absorbing different dye molecules. The corresponding mechanism is also discussed. The principle reported in this Letter can be used to fabricate full spectrum PDs with distinctive wavelength selectivity.