Observation of resonance electronic and non‐resonance‐enhanced vibrational natural Raman optical activity

Natural resonance electronic Raman optical activity (ROA) is observed for the first time. Coincidently, the first example of vibrational ROA enhanced by low‐lying electronic transition is reported. These new phenomena were measured using the rare‐earth complex Eu(tfc)₃ (+)‐tris[3‐trifluoroacetyl‐D ‐camphorato]europium(III), where electronic resonance occurs between the 532‐nm laser excitation and the ⁷F₁ → ⁵D₁ transition of the Eu³⁺ metal center. Electronic Raman spectra involve the Raman transitions terminating on the low‐lying electronic states of Eu(tfc)₃. The observed vibrational ROA spectra are enhanced relative to typical ROA spectra by the proximity of vibrational states of Eu(tfc)₃ to its low‐lying electronic states with significant magnetic‐dipole character, whereas the parent vibrational Raman spectra do not appear to be resonance‐enhanced since the 532‐nm vibrational Raman spectrum has similar relative intensities to the corresponding Raman spectrum measured with 1064‐nm laser excitation. Copyright © 2010 John Wiley & Sons, Ltd.     

SERS activity of pulsed laser ablated silver thin films with controlled nanostructure

The surface‐enhanced Raman scattering (SERS) activity of silver thin films deposited by the pulsed laser ablation technique was investigated. The samples were grown in a controlled Ar atmosphere at pressures ranging between 10 and 70 Pa, and changing the number of laser pulses. Different surface morphologies, from isolated nearly spherical nanoparticles (NPs) to larger islands with smooth edges, were observed by means of scanning and transmission electron microscopies, as a function of the different deposition conditions adopted. SERS measurements were performed by soaking the samples in rhodamine 6G aqueous solutions over the concentration range between 1.0 × 10⁻⁴ and 5.0 × 10⁻⁸ M . Raman spectra were acquired using both the 632.8 and 514.5 nm excitation sources. The dependence of the SERS activity of the samples on the observed surface morphology is presented and discussed. The presence of the so called hot spots is envisaged. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of Langmuir-Blodgett polydiacetylene polymer films by surface enhanced Raman scattering

Large shifts (～50 cm^-1) to lower energies in the C=C and C≡C stretching modes of the Raman spectra of Langmuir-Blodgett deposted thin films of 15-8 polydiacetylene polymer are observed as the film thickness changes from a monolayer or one bilayer to two or more bilayers. The spectra of two or more bilayers are found to be identical to bulk material. These large shifts, observed by surface enhanced Raman scattering from the silver island substrate on which the films are deposited are attributed to delocalization of the electronic states in the backbone of the polydiacetylene polymer.     

Study of Raman spectrum of uranium using a surface‐enhanced Raman scattering technique

Raman spectroscopic investigation on weak scatterers such as metals is a challenging scientific problem. Technologically important actinide metals such as uranium and plutonium have not been investigated using Raman spectroscopy possibly due to poor signal intensities. We report the first Raman spectrum of uranium metal using a surface‐enhanced Raman scattering‐like geometry where a thin gold overlayer is deposited on uranium. Raman spectra are detected from the pits and scratches on the sample and not from the smooth polished surface. The 514.5‐ and 785‐nm laser excitations resulted in the Raman spectra of uranium metal whereas 325‐nm excitation did not give rise to such spectra. Temperature dependence of the B_3g mode at 126 cm⁻¹ is also investigated. Copyright © 2011 John Wiley & Sons, Ltd.     

Nanostructuring of thin Au films by means of short UV laser pulses

The particle size distribution, morphology and optical properties of the Au nanoparticle (NP) structures for surface enhanced Raman signal (SERS) application are investigated in dependence on their preparation conditions. The structures are produced from relatively thin Au films (10–20 nm) sputtered on fused silica glass substrate and irradiated with several pulses (6 ns) of laser radiation at 266 nm and at fluencies in the range of 160–412 mJ/cm². The SEM inspection reveals nearly homogeneously distributed, spherical gold particles. Their initial size distribution of the range of 20–60 nm broadens towards larger particle diameters with prolonged irradiation. This is accompanied by an increase in the uncovered surface of the glass substrate and no particle removal is observed. In the absorption profiles of the nanostructures, the broad peak centred at 546 nm is ascribed to resonant absorption of surface plasmons (SPR). The peak position, halfwidth and intensity depend on the shape, size and size distribution of the nanostructured particles in agreement with literature. From peak intensities of the Raman spectra recorded for Rhodamine 6G in the range of 300–1800 cm⁻¹, the relative signal enhancement by factor between 20 and 603 for individual peaks is estimated. The results confirm that the obtained structures can be applied for SERS measurements and sensing.     

Elimination of oxidation and decomposition by SnCl2 in the SERS study of pyridoxine on a roughened Au electrode

Severe interference from the oxidation and laser carbonization was encountered in the measurements of surface‐enhanced Raman scattering (SERS) spectra of pyridoxine (PN) on the roughened gold electrode. However, we found that high‐quality SERS spectra of PN could be obtained by the introduction of SnCl₂, which only has few Raman peaks at low wavenumbers. SnCl₂, as a good reductive, is capable of removing the oxidative species on the gold substrate and the dissolved oxygen in solution, and as a result lowering the open circuit potential (OCP). Sn(II) can also strongly chemically adsorb on the gold surface and interact with PN through coordination/chelation, such that not only to prevent PN from damage by the giant electromagnetic field for the ‘first‐layer’ effect, but also to give rise to very strong Raman scattering signals of PN where chemical enhancement plays an important role. Those are the main reasons for the elimination of the oxidation and decomposition of PN and for the high‐quality SERS spectra of PN. The way the SnCl₂ confines PN within the enhanced electromagnetic field by its ability of adsorption and coordination/chelation can be utilized to improve the routine SERS analysis of analogous type of reactive organic/biomolecules. In addition, this method has been successfully extended to the SERS measurements of PN on the substrates of roughened silver and copper. Copyright © 2008 John Wiley & Sons, Ltd.     

Laser-induced decomposition of sodium azide

The decomposition of sodium azide, NaN₃, has been studied using a pulsed carbon dioxide laser. Chemiluminescence associated with the decomposition has been measured with and, without, the azide surface in the detector field of view. Near the decomposition threshold, emission has been observed in the wings of the Na 3p → 3s transition at 589 nm that has not been observed in previous azide decomposition experiments. This emission may be a result of surface reactions. Raman and visible absorption spectra have been obtained for the irradiated material; a new, as yet unidentified, band was observed in the Raman spectrum.     

Synthesis by pulsed laser ablation in Ar and SERS activity of silver thin films with controlled nanostructure

Thin silver films were deposited by pulsed laser ablation in a controlled Ar atmosphere and their SERS activity was investigated. The samples were grown at Ar pressures between 10 and 70 Pa and at different laser pulse numbers. Other deposition parameters such as laser fluence, target to substrate distance and substrate temperature were kept fixed at 2.0 J/cm², 35 mm and 297 K. Film morphologies were investigated by scanning and transmission electron microscopies (SEM, TEM). Surface features range from isolated nearly spherical nanoparticles to larger islands with smoothed edges. Cluster growth is favored by plume confinement induced by background gas. After landing on the substrate clusters start to aggregate giving rise to larger structures as long as the deposition goes on. Such a path of film growth allows controlling the surface morphology as a function of laser pulse number and Ar pressure. These two easy-to-manage process parameters control the number density and the average size of the as-deposited nanoparticles. We investigated the influence of substrate morphologies on their surface enhanced Raman scattering properties. Raman measurements were performed after soaking the samples in rhodamine 6G aqueous solutions over the concentration range between 1.0 × 10⁻⁴ and 5.0 × 10⁻⁸ M. The sensitivity of the film SERS activity on the surface features is put into evidence.     

Investigation of inverse Raman scattering using the method of intra-cavity spectroscopy

The detectability of Raman absorption lines is enhanced by inserting the Raman sample into the cavity of a broad-band dye laser where the sample is simultaneously pumped by monochromatic radiation. With this technique we were able to detect Raman samples with good scattering efficiencies in a concention of 10^?3 mole/? or to obtain nearly complete Raman spectra within a time interval of 30 nsec.Exposures in which the dye laser radiation and the strong monochromatic radiation are polarized parallel or perpendicularly to each other are compared with spontaneous Raman spectra which are polarized parallel or perpendicularly.     

Raman and surface-enhanced Raman spectroscopy evidence for oxidation-induced decomposition of graphite

It has been proposed that reduction of exfoliated graphite oxide could be a potential method for producing large quantities of graphene. Raman and surface-enhanced Raman spectroscopy are used to show that oxidation of graphite and exfoliated graphite significantly increases the defect structure of both materials. This would likely lead to a heavily defected graphene structure when oxygen is removed. To insure the observed decomposition is not due to the laser light, the effect of laser intensity on the materials was investigated. It was found that at the highest laser intensity (1.4 × 10⁸ W/M²) there was a significant increase in defects. However, lower laser intensity was found which did not produce defects and was used in the studies of the effect of oxidation on the spectra.     

Direct observation of laser-induced crystallization of a-C:H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm². At fluences below 100 mJ/cm² an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm². The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.     

Synthesis of DLC films by PLD from liquid target and dependence of film properties on the synthesis conditions

DLC (Diamond-like carbon films) were prepared by pulsed laser ablation of a liquid target at substrate temperatures from 18 to 600°C using 248 nm KrF excimer laser. The sp³ hybridization state carbon formation was additionally promoted by gaseous H₂O₂ flow through the reaction chamber and substrate excitation by the same laser beam. Deposited DLC films were characterised by Raman scattering spectroscopy and atomic force microscopy (AFM). Comparative AFM and Raman study shows that the increase in the content of sp³ type bonding in DLC is in correlation with the increase of the surface roughness of the samples prepared.     

Theoretical elucidation of the origin of surface‐enhanced Raman spectra of PCB52 adsorbed on silver substrates

To better understand experimentally observed surface‐enhanced Raman Scattering (SERS) of polychlorinated biphenyls (PCBs) adsorbed on nanoscaled silver substrates, a systematic theoretical study was performed by carrying out density functional theory and time‐dependent density functional theory calculations. 2,2′,5,5′‐tetrachlorobiphenyl (PCB52) was chosen as a model molecule of PCBs, and Ag_n (n = 2, 4, 6, and 10) clusters were used to mimic active sites of substrates. Calculated normal Raman spectra of PCB52–Ag_n (n = 2, 4, 6, and 10) complexes are analogical in profile to that of isolated PCB52 with only slightly enhanced intensity. In contrast, the corresponding SERS spectra calculated at adopted incident light are strongly enhanced, and the calculated enhancement factors are 10⁴ ~ 10⁵. Thus, the experimentally observed SERS phenomenon of PCBs supported on Ag substrates should correspond to the SERS spectra rather than the normal Raman spectra. The dominant enhancement in Raman intensities origins from the charge transfer resonance enhancement between the molecule and clusters. Copyright © 2014 John Wiley & Sons, Ltd.     

Temperature‐dependent Raman study of PrFeO3thin film

Low‐temperature Raman study of (001)‐oriented PrFeO₃ thin film of around 200 nm thickness deposited on a LaAlO₃ (001) substrate by using the pulsed‐laser deposition technique is presented. X‐ray diffraction analysis of this film shows an orthorhombic structure with Pbnm space group. The observed substrate‐induced strain is found to be small. In the room temperature Raman spectra, different Raman modes were observed that were classified according to the orthorhombic structure. All the observed modes show a decrease in wavenumber with rise in temperature, except the B_1g mode (624 cm⁻¹) which shows some anomalous behavior. We tried to correlate the variations in linewidth and position with temperature for the observed modes with the octahedral disorder of FeO₆. Many possibilities are presented to explain the observed results. Copyright © 2010 John Wiley & Sons, Ltd.     

Generation of nanospikes via laser ablation of metals in liquid environment and their activity in surface-enhanced Raman scattering of organic molecules

The formation of dense arrays of nanospikes occurs under laser ablation of bulk targets (Ag, Au, Ta, Ti) immersed in liquids such as water or ethanol. The average height of spikes is 50 nm and their density on the target amounts to 10¹⁰ cm⁻². The effect is observed with sufficiently short laser pulses. In particular, either a 350 ps or a 90 ps Nd:YAG lasers are used in their fundamental harmonics. The nanospikes are characterized by UV-Visible reflection spectrometry and atomic force microscopy. The oscillations of electrons within nanospikes result in a permanent coloration of the surface and a modification of the optical reflection spectra of the metal. Scanning the laser beam along the metal surface allows its nanostructuring over extended areas (∼1 cm²). The nanostructured Ag surface shows enhanced Raman scattering of acridine molecules at a concentration of 10⁻⁵ M/l, whereas the initial Ag targets do not show any signal within the accuracy of measurements.     

Nanoporous Ag–GaN thin films prepared by metal‐assisted electroless etching and deposition as three‐dimensional substrates for surface‐enhanced Raman scattering

Three‐dimensional (3D) nanoporous gallium nitride (PGaN) scaffolds are fabricated by Pt‐assisted electroless hydrofluoric acid (HF) etching of crystalline GaN followed by in situ electroless deposition of Ag nanostructures onto the interior surfaces of the nanopores, yielding a large surface area substrate for surface‐enhanced Raman scattering (SERS). The resulting 3D SERS‐active substrates have been optimized by varying reaction parameters and starting material concentration, exhibiting enhanced Raman signals 10–100× more intense than either (1) sputtered Ag‐coated porous GaN or (2) Ag‐coated planar GaN. The increase in SERS signal is attributed to a combination of the large surface area and the inherent transparency of PGaN in the visible spectral region. Overall, Ag‐decorated PGaN is a promising platform for high sensitivity SERS detection and chemical analysis, particularly for reaction and metabolic products that can be trapped inside the highly anisotropic nanoscale pores of PGaN. The potential of this sampling mode is illustrated by the ability to acquire Raman spectra of adenine down to 5 fmol. Additionally, correlated SERS and laser desorption/ionization mass spectrometry spectra can be acquired from same sample spot without further preparation, opening new possibilities for the investigation of surface‐bound molecules with substantially enhanced information content. Copyright © 2012 John Wiley & Sons, Ltd.     

Generation of Ag nanospikes via laser ablation in liquid environment and their activity in SERS of organic molecules

The formation of dense arrays of nanospikes occurs under laser ablation of bulk silver immersed in liquids such as water or ethanol. The average height of spikes is 50 nm and their density on the target amounts to 10¹⁰ cm^-2. This effect is observed with sufficiently short laser pulses. In particular, either a 350 ps or a 90 ps Nd:YAG lasers are used operating in their fundamental harmonics. These nanospikes are characterized by UV–visible reflection spectrometry and atomic force microscopy. The oscillations of electrons within Ag nanospikes results in a permanent coloration of the surface and a modification of the optical reflection spectra of the metal. Nanospikes show a peak of plasmon resonance around 380 nm, which shifts to the visible range upon oxidation in air. The initial spectrum may be restored by reduction of the oxidized Ag surface through processing in an ammonia aqueous solution. Scanning the laser beam along the metal surface allows its nanostructuring over extended areas (∼1 cm²). The nanostructured Ag surface shows enhanced Raman scattering of acridine molecules at a concentration of 10^-5 M/l, whereas initial Ag target do not show any signal within the accuracy of measurements. PACS 68.65.k; 64.70.Dv; 79.20.Ds; 42.62.Cf     

Raman spectroscopy of the OH group vibrations in structural complexes of liquid water

A number of spectral features in the structure of the OH band of water located at ～3450 cm^?1 are registered in a series of 34 Raman spectra. Each spectrum of the series is excited by a pair of successive secondharmonic pulses of a moderate intensity (15–20 MW/cm²) from a Nd:YAG laser operating at a pulse repetition frequency of 1 Hz. The pairs of pulses in the series follow at an interval of 15–20 s. It is found that, in ten measurements of the first series, there occur three successively reproduced spectra with an anomalously narrow band (down to 260 cm^?1) and spectra with a local minimum in the range of 3400 cm^?1. The occurrence of the latter spectra was predicted previously as a manifestation of the occurrence of structural fragments of water. In the range 3690–3700 cm^?1, a narrow peak of free OH vibrations is observed, whose intensity decreases in subsequent series. Some Raman spectra are similar to the spectra of the OH vibrations of water clusters in molecular beams and surface layers. These data make it possible to assume that structural complexes exist in the volume of water irradiated by the laser beam, with the lifetime of some of them being no shorter than 40 s.     

Structure and conformation of Arg8 vasopressin modified analogs

Fourier‐transform Raman and infrared spectra were acquired for four arginine vasopressin (AVP) analogs containing L ‐diphenylalanine (Dpa): [Dpa²]AVP, [Cpa¹,Dpa²]AVP, [Dpa³]AVP, and [Cpa¹,Dpa³]AVP (where Cpa denotes 1‐mercaptocyclohexaneacetic acid). We compared and analyzed these spectra. In addition, the Raman spectra were compared to the corresponding surface‐enhanced Raman scattering spectra recorded in an aqueous silver colloidal dispersion. Silver colloidal dispersions prepared by the simple borohydride reduction of silver nitrate were used as substrates. The geometry of these molecules etched on the silver surface was deduced from the observed changes in the intensity enhancement, breadth, and shift in wavenumber of the Raman bands in the spectra of the bound versus free species. Based on the obtained data, adsorption mechanisms were proposed for each case, and the suggested adsorbate structures were compared. All the molecules were thought to adsorb onto a silver surface via a phenyl ring, free electron pairs on the sulfur atom, and CO and  CONH‐bonds. However, the orientation of these fragments on the colloidal silver surface and the strength of the interactions with this surface are different. For [Dpa³]AVP and [Cpa¹,Dpa³]AVP, a strong interaction among the—CCN‐peptide fragment and the colloidal silver surface occurs. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering within silver‐nanoparticle‐decorated nanometric apertures

An analytical approach using enhanced Raman spectroscopy to record molecular vibrations and associated molecular images within nanometric apertures is presented, which can essentially rival or surpass its counterparts, i.e. fluorescence microscopy, by providing unique structure‐specific information forward to chemical identification and structure elucidation. Utilizing a precise nanolithographic technology and the following chemically electroless silver deposition procedure, we deliberately construct the large scale zero‐mode waveguide array in gold film with embossed silver nanostructures on the bottom of nanowells capable of acquiring enhanced Raman spectra with substantial sensitivity and high chemical fidelity. Two chemicals, aminothiophenol (4‐ATP) and Rhodamine 6G, respectively, are employed as molecular indicators to successfully demonstrate the capability of this analytical strategy by exhibiting high‐quality Raman spectra and 2D chemical‐specific images. With a high magnitude objective (60×), we enable to acquire Raman spectra from a single nanometric aperture and quantitatively determine a peak enhancement factor of 3.63 × 10⁵ for ATP, while 1.25 × 10⁶ to Rhodamine 6G, comparable with a regular nanoparticle‐based surface‐enhanced Raman spectroscopy‐active substrate. Overall, the compelling characteristics of this detection scheme highlight its privileges for interrogating the individual molecular behavior in extremely confined geometry and illustrating the chemical insights of trace components without any labeling reagent and extra sample preparation. Copyright © 2013 John Wiley & Sons, Ltd.