Model aluminum-poly(p-phenylenevinylene) interfaces studied by surface raman spectroscopy

Polymeric organic light-emitting diodes (OLEDs) have emerged as inexpensive and versatile alternatives to traditional inorganic-based display technologies. Further advances in this field depend on extending device lifetimes and improving electroluminescence efficiencies, both of which are strongly coupled to the integrity of the electrode/organic contacts. Therefore, the deposition of low-work function metals on organic materials has been extensively studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and near-edge X-ray adsorption fine structure (NEXAFS) to ascertain information about the chemical and electronic states within the interfacial region. However, to date, the only molecular structural information about metal-organic species formed in these regions has come from fits of theoretical models to existing XPS and UPS data. Very little direct structural information exists about the potential multitude of metal-organic species formed during cathode deposition. We report the use of surface Raman spectroscopy to study the interactions between aluminum and trans-stilbene, a model for poly(p-phenylenevinylene) (PPV). The Raman spectral data suggest preferential formation of covalent Al-C bonds at the vinylene carbons as opposed to the phenyl carbons of PPV.     

Dynamics of vacancy defects in 2-D crystalline monolayers under chiral smectic thin films studied by scanning tunnelling microscopy

Scanning tunnelling microscopy (STM) can be used to image adsorbed organic molecules in real space and real time. The technique seems especially well suited for imaging 2-D crystalline monolayers formed under liquid crystal films. In addition to observing perfect 2-D crystals, STM provides the ability to observe crystal defects, and to observe how these defects evolve over time. In this study two different vacancy defects in 2-D lamellar monolayers of chiral liquid crystal molecules under bulk smectic films were observed in situ. Both vacancies showed dynamic behaviour and an unexpected transport anisotropy.     

Electrolyte—formamide interactions studied by raman spectroscopy

The effects of dissolved electrolytes on the structure of liquid formamide have been investigated by Raman spectroscopy. The spectral features are dependent on the concentration and nature of the dissolved electrolyte and are discussed in terms of direct electrolyte formamide interactions. Two vN—H bands, arising from ion—formamide species, have been observed superimposed on the spectrum of residual liquid formamide. The results lend further support to the interaction model for electrolytes in liquid formamide previously proposed by us on the basis of nuclear magnetic resonance and infrared spectral data.     

Surfactant enhanced lipase containing films characterized by confocal laser scanning microscopy

An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM–EDXA, UV–vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that –OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV–vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5 ± 2.5 nm and 7.8 ± 2.3 nm; while Pt nanoparticles were in the size range of 2.4 ± 0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles.     

Investigation of alignment mechanism of liquid crystal on polyimide Langmuir-Blodgett films by scanning tunnelling microscopy

The homogeneous alignment of a liquid crystal material, 4'-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.     

Investigation of alignment mechanism of liquid crystal on polyimide Langmuir-Blodgett films by scanning tunnelling microscopy

Abstract

The homogeneous alignment of a liquid crystal material, 4′-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.     

Surface enhanced raman spectroscopy of the non-aqueous system silver/acetonitrile,KSCN

The ν₁(CN) band of SCN^? in acetonitrile solvent has been observed, surface enhanced, at a silver electrode. Adsorption through the S of SCN^? is suggested. The changes of frequency and intensity with the change of electrode potential are reported. The υ(CN) band of the solvent is also probably enhanced.     

茶叶浸出液中农药残留表面拉曼光谱检测与初步定量分析

选取甲基对硫磷和水胺硫磷为研究对象,改良了传统的QuEChERS前处理工艺,以自制纳米金溶胶为增强基底,利用表面增强拉曼光谱(SERS)技术,对茶叶浸出液中的农药残留进行检测。通过比对两种有机磷农药的拉曼特征峰进行定性分析。同时,选取570,1034,1107和1202 cm^-1等拉曼位移附近的特征峰光谱数据,利用微分等数学手段,结合偏最小二乘法(PLSR)建立回归方程,预测样品中农药残留含量。所得预测数值与气相色谱-质谱联用(GC-MS)法检测值对比,验证本方法的可行性与可信度。结果表明:基于SERS技术对上述两种有机磷农药的检出限可达0.05 mg/L;通过数学模型分析建立回归方程,其线性相关系数范围为0.9077~0.9824,预测均方根误差(RMSEP)范围为0.77%~2.68%;利用回归方程得到的预测值与GC-MS检测结果基本接近,相对误差范围-5.16%~9.03%,回收率为81.4%~115.1%,说明可以用SERS技术对茶叶浸出液中的有机磷农药残留进行定性和初步定量分析。     

Rotational tunnelling of methane on the surface of graphitized carbon blacks studied by neutron scattering spectroscopy

Rotational tunnelling transitions of methane adsorbed on the surface of graphitized carbon black have been observed by incoherent neutron inelastic scattering. Two transitions have been observed, at 58 μeV and 108 μeV, which arise from the barrier to rotation about axes parallel to the surface. The methane molecules undergo rotational diffusion about an axis perpendicular to the surface. The tunnelling transitions are quite sharp at 6 K but become broader as the temperature is raised. At 30 K the motion is best described as isotropic rotational diffusion. The tunnelling spectrum has been observed on two different graphitized carbon blacks, Vulcan III (71 m² g^?1) and Sterling FT (11 m² g^?1). the latter has a more homogeneous surface and gives a sharper spectrum. Tunnelling has also been observed for methane adsorbed on Sterling FT preplated with a monolayer of xenon. When a second layer of methane is added to the first the tunnelling spectrum becomes broad and an inelastic feature appears at higher energy corresponding to a slightly hindered rotation. The value of using tunnelling spectra of adsorbed species to test models of surface forces is discussed with examples of different atom—atom potential parameters.     

Interaction of proflavine with DNA studied by colloid surface enhanced resonance Raman spectroscopy

The interaction of the mutagenic highly fluourescing proflavine (3,6-diaminoacridine: PF) dye with calf thymus DNA has been studied by Surface Enhanced Resonance Raman Scattering (SERRS). Since the Ag-colloids almost completely quenche the strong fluorescence it is possible to obtain excellent vibrational spectra in a wide frequency range providing valuable information about the intercalation. The intercalation does not affect the vibrational frequencies of the proflavine dye. On the other hand, intensity changes are observed in some of the ring- and NH₂-modes of proflavine upon intercalation. This Raman hypochromism is characteristic for ring stacking interactions and in the SERRS spetroscopy for an additional effects of the dye orientation to the surface.     

Electrochemical processes of meso-tetrakis (4-sulfonatophenyl) porphine at a silver electrode studied by surface-enhanced resonance raman spectroscopy

The potential dependence of surface-enhanced resonance Roman spectra of meso-tetrakis (4-sulfonatophenyl) porphine (TSPP) in 0.05 M H₂SO₄ reveals two electrochemical processes at a silver electrode surface. One which begins around ?0.3 V is interpreted as dissociation of aggregated TSPP to monomers. The other which occurs near ?0.4 V is ascribed to a partial Ag incorporation of TSPP molecules.     

Photoinduced coupling and adsorption of caffeic acid on silver surface studied by surface-enhanced Raman spectroscopy

The effect of light on the caffeic acid (CA) oxidative coupling is studied in aqueous solution and on silver by surface-enhanced Raman spectroscopy (SERS). CA can polymerize in aqueous solution or on a metal surface through an oxidative mechanism involving the formation of the corresponding quinone giving rise to characteristic Raman features in each case. We show here that the effect of light in relation to this oxidative coupling is crucial taking place mainly in the solution bulk. The products derived from such polymerization can then adsorb on the silver surface employed for SERS measurements, thus allowing its detection by Raman spectroscopy. The influence of irradiation time and the wavelength of the light employed for the photoinduced coupling was investigated.     

2-D self-assembly of the bis(phthalocyaninato)terbium(III) single-molecule magnet studied by scanning tunnelling microscopy

The presence of alkoxyl chains on the single-molecule magnet [(Pc')2Tb(III)] and its planar character favour its 2-D self-assembly on HOPG with the molecular plane parallel to the graphite surface, enabling us to control and/or predict the orientation of the preferential magnetization axis on the surface.     

Thin liquid films studied by atomic force microscopy

Since its invention twenty years ago the atomic force microscope (AFM) has become one of the most important instruments in colloid and interface science. The ability of tracing force profiles between single particles or particles and flats in liquid environment makes it a tool-of-choice for investigating thin liquid films. In this paper we review experimental work on confined Newtonian and non-Newtonian liquids using the AFM.     

Observations of image contrast and dimerization of decacyclene by low temperature scanning tunneling microscopy

Low temperature scanning tunneling microscopy studies revealed both monomer and dimer forms of decacyclene (DC) on atomically clean Cu(100) and Cu(111). The observed image contrast in DC is strongly bias dependent and also influenced by tip modifications. Alternatively, dimers appear solely as protrusions and are nearly bias independent. We provide evidence of both dimer formation and dissociation and suggest that two DC molecules stack by aligning their molecular planes in a parallel fashion with respect to the surface. Dimers and their surface-dependent properties demonstrate the interplay between surface-molecule and molecule-molecule interactions.     

Layered self-organized structures on poly(3-octylthiophene) thin films studied by scanning probe microscopy

The morphology and mechanical properties of poly-(3-octylthiophene) P3OT films thin has been studied by scanning force microscopy techniques. On these films we find self-assembled layered structures that appear regardless of the preparation conditions, that is, spin-coating or drop-casting, of the solvent concentration or of the type of substrate. Using the drop-casting method for sample preparation these layered structures are hardly visible due to the high surface roughness, while using spin-coating these structures are the main topographic feature on the surface. These structures have typically one or two layers, even though occasionally up to four layers have been observed. Each layer has a height of 4-5 nm, which is associated to crystalline P3OT domains and lay on the polymer film. The size of these structures increases with increasing concentration of the P3OT in the solvent. We find well differentiated morphological, electrostatic as well as mechanical properties for the self-assembled structures as compared to the rest of the polymer film. Finally, the growth rate of these structures has been studied.     

Non aggregated colloidal silver nanoparticles for surface enhanced resonance Raman spectroscopy

Silver nanoparticles with a tuneable λ max were produced as colloids by heterogeneous nucleation. The synthesis process is both fast and repeatable, producing stable PVA capped nanoparticles. The colloid's effectiveness in the SERRS system was investigated using Rhodamine 6G, R6G, Crystal Violet, CV, and Malachite Green, MG, as probe molecules. A clear sensing trend was observed, where the Raman signal emitted was significantly enhanced by the addition of silver nanoparticles. A build up of signal intensity is observed until an optimum ratio is achieved, followed by a decline in signal intensity as the concentration of nanoparticles is further increased. The sensing trend appeared to be dependant on the structure of these model molecules with similarly structured compounds exhibiting similar trends. Thus a maximum enhancement with the Ag: analyte molar ratio of ～ 5.56: 1, was seen for CV and MG whereas R6G had a maximum enhancement at the Ag: analyte molar ratio of ～ 2.25: 1.     

Thermal stability of surface freezing films in Ga-based alloys: an x-ray photoelectron spectroscopy and scanning tunneling microscopy study

We have investigated the thickness and surface structure of surface freezing films in Ga-Bi and Ga-Pb alloys over a wide temperature range between room temperature and the respective surface freezing transitions by x-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). For the example of a Ga-Bi alloy dilute in Bi, XPS measurements show that the surface freezing film has a nearly constant value of approximately 25 A between the surface freezing temperature of 130 degrees C and room temperature if the sample is cooled slowly (5 Kh). On heating to 130 degrees C the film thickness exhibits a clear hysteresis on melting. On quenching the alloy sample (>100 Kh) the film thickness increases by almost a factor of 10. These observations indicate that the surface freezing films are metastable. The surface structure of the surface freezing films of various Ga-rich Ga-Bi and Ga-Pb alloys has been probed for the first time by STM at different temperatures below and above the bulk eutectic point. Atomically resolved STM images show the surface structures of pure Bi (0001) and Pb (111), respectively, at room temperature. On heating above the eutectic temperature the surface structure of the films does not change significantly as judged from the size and thickness of Pb or Bi terraces. These observations together with the film thickness variation with temperature indicate that the surface freezing films behave like a metastable independent surface phase. These results together with the wetting characteristics of these alloys suggest that surface freezing in these systems is a first order surface phase transition between wetting and metastable surface freezing films. The energy barrier for nucleation is strongly reduced due to a lowering of the interfacial energy if the nucleus is completely immersed in the respective wetting layer.