Benzenethiol Molecules Research Articles

Paper‐based surface‐enhanced Raman spectroscopy sensors for field applications

AbstractPaper‐based surface‐enhanced Raman spectroscopy (SERS) sensors can be fabricated easily by dropcasting or inkjet printing colloidal Au nanoparticles onto cellulose‐based filter papers. They are flexible, economical, and sensitive and provide the crucial advantage of point‐of‐need sampling for application in the field. In this study, paper‐based SERS sensors are fabricated through inkjet printing of a colloidal Au sol onto a filter paper substrate. We have characterized their SERS performances with benzenethiol and pyridine molecules using a handheld Raman analyzer. Due to the heterogeneous loading of the Au nanoclusters on the paper substrate, we introduce the concept of receiver operating characteristic as an alternate measurand to quantify the performance of these sensors. With their inherent filtration sampling capability, we demonstrate the use of paper SERS sensors for the detection of chemical aerosols. Lastly, we present the use of a precision materials printer to deposit quantifiable amounts of analyte (fentanyl) uniformly across the active sensing area of a paper SERS sensor. This will allow for analyte‐loaded certified references to be prepared and used in the field as standards for comparison.

Temperature-Dependent Charge Transport of Large-Area Molecular Junctions with PEDOT:PSS Electrodes

We report on the temperature-dependent transport behaviors of large-area molecular junctions fabricated with poly-(3,4-ethylene-dioxythiophene) stabilized with polystyrene sulphonic acid (PEDOT:PSS) interlayer electrodes and the archetypal benzenethiol molecules. In this study, we investigated two different benzenethiol molecules: 4-methylbenzenethiol (MBT) and 1,4-benzenedithiol (BDT), which have the identical backbone structure but different top end-groups. The charge transport through the molecular junctions was dominated by distinct interfacial contact properties between the PEDOT:PSS electrodes and the component molecules. We also observed that the electrical characteristics of the MBT junctions are influenced by the PEDOT grain size, particularly depending on the annealing temperature.

Nanoscale junctions for single molecule electronics fabricated using bilayer nanoimprint lithography combined with feedback controlled electromigration

Nanoimprint lithography (NIL) is a fast, simple and high throughput technique that allows fabrication of structures with nanometre precision features at low cost. We present an advanced bilayer nanoimprint lithography approach to fabricate four terminal nanojunction devices for use in single molecule electronic studies. In the first part of this work, we demonstrate a NIL lift-off process using a bilayer resist technique that negates problems associated with metal side-wall tearing during lift-off. In addition to precise nanoscale feature replication, we show that it is possible to imprint micron-sized features while still maintaining a bilayer structure enabling an undercut resist structure to be formed. This is accomplished by choosing suitable imprint parameters as well as residual layer etching depth and development time. We then use a feedback controlled electromigration procedure, to produce room-temperature stable nanogap electrodes with sizes below 2 nm. This approach facilitates the integration of molecules in stable, solid-state molecular electronic devices as demonstrated by incorporating benzenethiol as molecular bridges between the electrodes and characterizing its electronics properties through current–voltage measurements. The observation of molecular transport signatures, showing current suppression in the I–V behaviour at low voltage, which is then lifted at high voltage, signifying on- and off-resonant transport through molecular levels as a function of voltage, is confirmed in repeated I–V sweeps. The large conductance, symmetry of the I–V sweep and small value of the voltage minimum in transition voltage spectroscopy indicates the bridging of the two benzenethiol molecules is by π–stacking.

Charge Transport Characteristics of the PEDOT:PSS Interlayer Electrode/Molecule Physisorbed Contact in Large-Area Molecular Junctions.

Here we studied the effect of electrical contact between the PEDOT:PSS conducting polymer and the prototype benzenethiol molecules on the charge transport through the PEDOT:PSS-electrode molecular junctions. For this study, two different benzenethiol molecules were incorporated into the molecular junctions: 4-methylbenzenethiol (MBT) and 1,4-benzenedithiol (BDT). They have the same backbone structure but different top end-group. By investigating the MBT and BDT junctions with the PEDOT:PSS electrode, we demonstrated that the charge transport characteristics were dominated by distinct electrical contact properties at the PEDOT:PSS/molecule interface.

Electrical transport characterization of benzenedithiol and methylbenzenethiol molecular junctions with PEDOT:PSS interlayer electrodes

We have fabricated and characterized molecular electronic devices using benzene-based molecular monolayers and PEDOT:PSS (3,4-ethylenedioxythiophene) interlayer electrodes. In particular, we compared the electrical characteristics of two different benzenethiol molecules, 4-methylbenzenethiol (MBT) and 1,4-benzenedithiol (BDT), which have the same backbone structure but different top-end groups. From statistically representative data, we observed that the current density of the MBT devices was greater than that of the BDT devices by a factor of ∼4. The difference in current density between the two molecular electronic devices was attributed to the difference in the contact properties between the MBT and BDT molecules with the PEDOT:PSS interlayer electrodes. We also found an increase in the current density with increasing temperature within the range of 300 − 400 K, which could be caused by changes in the interfacial properties of the PEDOT:PSS-molecule contact and by removal of residual solvent or water from the interface.

Structural changes in a Schiff base molecular assembly initiated by scanning tunneling microscopy tip

We report the controlled self-organization and switching of newly designed Schiff base (E)-4-((4-(phenylethynyl) benzylidene) amino) benzenethiol (EPBB) molecules on a Au (111) surface at room temperature. Scanning tunneling microscopy and spectroscopy (STM/STS) were used to image and analyze the conformational changes of the EPBB molecules. The conformational change of the molecules was induced by using the STM tip while increasing the tunneling current. The switching of a domain or island of molecules was shown to be induced by the STM tip during scanning. Unambiguous fingerprints of the switching mechanism were observed via STM/STS measurements. Surface-enhanced Raman scattering was employed, to control and identify quantitatively the switching mechanism of molecules in a monolayer. Density functional theory calculations were also performed in order to understand the microscopic details of the switching mechanism. These calculations revealed that the molecular switching behavior stemmed from the strong interaction of the EPBB molecules with the STM tip. Our approach to controlling intermolecular mechanics provides a path towards the bottom-up assembly of more sophisticated molecular machines.

Particle-on-Film Gap Plasmons on Antireflective ZnO Nanocone Arrays for Molecular-Level Surface-Enhanced Raman Scattering Sensors.

When semiconducting nanostructures are combined with noble metals, the surface plasmons of the noble metals, in addition to the charge transfer interactions between the semiconductors and noble metals, can be utilized to provide strong surface plasmon effects. Here, we suggest a particle-film plasmonic system in conjunction with tapered ZnO nanowire arrays for ultrasensitive SERS chemical sensors. In this design, the gap plasmons between the metal nanoparticles and the metal films provide significantly improved surface-enhanced Raman spectroscopy (SERS) effects compared to those of interparticle surface plasmons. Furthermore, 3D tapered metal nanostructures with particle-film plasmonic systems enable efficient light trapping and waveguiding effects. To study the effects of various morphologies of ZnO nanostructures on the light trapping and thus the SERS enhancements, we compare the performance of three different ZnO morphologies: ZnO nanocones (NCs), nanonails (NNs), and nanorods (NRs). Finally, we demonstrate that our SERS chemical sensors enable a molecular level of detection capability of benzenethiol (100 zeptomole), rhodamine 6G (10 attomole), and adenine (10 attomole) molecules. This work presents a new design platform based on the 3D antireflective metal/semiconductor heterojunction nanostructures, which will play a critical role in the study of plasmonics and SERS chemical sensors.

Templated fabrication of periodic arrays of metallic and silicon nanorings with complex nanostructures

Here we report a scalable colloidal templating approach for fabricating periodic arrays of metallic and silicon nanorings with complex nanostructures. Non-close-packed monolayer silica colloidal crystal prepared by a simple spin-coating technology is first used as template for making periodic arrays of mushroom-like composite nanostructures consisting of silica spherical caps and polymer stems. Subsequent metal sputtering and reactive ion etching lead to the formation of ordered asymmetric nickel nanorings which can be further utilized as etching masks for patterning periodic arrays of symmetric silicon nanorings. Moreover, periodic arrays of metallic and silicon concentric double nanorings can be fabricated by using the asymmetric nickel nanorings as templates. We have also demonstrated that gold concentric double nanorings show strong surface-enhanced Raman scattering (SERS) with a SERS enhancement factor of ∼9.5 × 107 from adsorbed benzenethiol molecules. The SERS enhancement and the electric field amplitude distribution surrounding gold concentric double nanorings have been calculated by using finite element electromagnetic modeling. This new colloidal templating technique is compatible with standard microfabrication and enables wafer-scale production of a variety of periodic nanorings with hierarchical structures that could find important technological applications in plasmonic and magnetic devices.

Nanogap-enhanced infrared spectroscopy with template-stripped wafer-scale arrays of buried plasmonic cavities.

We have combined atomic layer lithography and template stripping to produce a new class of substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. Our structure consists of a buried and U-shaped metal-insulator-metal waveguide whose folded vertical arms efficiently couple normally incident light. The insulator is formed by atomic layer deposition (ALD) of Al2O3 and precisely defines the gap size. The buried nanocavities are protected from contamination by a silicon template until ready for use and exposed by template stripping on demand. The exposed nanocavity generates strong infrared resonances, tightly confines infrared radiation into a gap that is as small as 3 nm (λ/3300), and creates a dense array of millimeter-long hotspots. After partial removal of the insulators, the gaps are backfilled with benzenethiol molecules, generating distinct Fano resonances due to strong coupling with gap plasmons, and a SEIRA enhancement factor of 10(5) is observed for a 3 nm gap. Because of the wafer-scale manufacturability, single-digit-nanometer control of the gap size via ALD, and long-term storage enabled by template stripping, our buried plasmonic nanocavity substrates will benefit broad applications in sensing and spectroscopy.

Bare and protected sputtered-noble-metal films for surface-enhanced Raman spectroscopy

Sputtered silver and gold films with different surface morphologies have been prepared and coated with a benzenethiol self-assembled monolayer. Rough noble metal films showed strong Raman features assigned to adsorbed benzenethiol molecules upon irradiation over a wide energy range in the visible spectrum, which disclosed the occurrence of a significant surface-enhanced Raman scattering with maximal enhancement factors as high as 6×106. In addition, the adsorption of ethanethiol onto silver surfaces hinders their corrosion over days while preserving mostly intact enhancement properties of naked silver. This study may be applied to develop stable and efficient metalized probes for tip-enhanced Raman spectroscopy.

Structure of benzenethiol monolayer on Pt(1 0 0)

The first-principle technique has been employed to determine the structure of benzenethiol (BT) molecule, molecular chains, monolayers and the BT/Pt(100) system. Their potential structures have been proposed. CASTEP calculation shows that molecular chains and monolayer are self-assembly system. At the coverage of 0.50ML, the stable structure in BT monolayer almost stands upright on Pt(100) in the hollow site, which is in good agreement with the experiment. At the coverage of 0.33ML and 0.25ML, when BT monolayer adsorbed on Pt(100) three adsorption sites are all stable, and the best one is bridge site. The interaction between S atom and Pt surface is stronger than that between BT molecules in the adsorption system.

Toward Understanding the Influence of Intermolecular Interactions and Molecular Orientation on the Chemical Enhancement of SERS

Implementation of SERS as an analytical technique is limited because the factors that govern the enhancement of individual vibrational modes are not well understood. Although the chemical effect only accounts for up to two orders of magnitude enhancement, it can still have a significant impact on the consistency of chemical spectral signatures. We report on a combined theoretical and experimental study on the benzenethiol on silver and 4-mercaptophenol on silver systems. The primary and unique finding was that for the benzenethiol on silver system the inclusion of interaction between multiple benzenethiol analyte molecules was essential to account for the relative enhancements observed experimentally. An examination of the molecular orbitals showed sharing of electron density across the entire model of multiple benzenethiol molecules mediated by the metal atoms. The addition of multiple 4-mercaptophenol molecules to the theoretical model had little effect on the predicted spectra, and we attribute this to the fact that a much larger model is necessary to replicate the networks of hydrogen bonds. Molecular orientation was also found to affect the predicted spectra, and it was found that an upright position improved agreement between theoretical and experimental spectra. An analysis of the vibrational frequency shifts between the normal Raman spectrum of the neat compound and the SERS spectrum also suggests that both benzenethiol and 4-mercaptophenol are in an upright position.

Generalized Fabrication of Monolayer Nonclose-Packed Colloidal Crystals with Tunable Lattice Spacing

Here, we report a simple colloidal transfer technology that enables scalable fabrication of monolayer nonclose-packed silica colloidal crystals on a large variety of substrates. Two-dimensional colloidal crystals with an unusual nonclose-packed structure are first assembled on silicon wafers by a spin-coating technique. A poly(vinyl alcohol) (PVA) film cast upon the spin-coated colloidal crystal is used to transfer the nonclose-packed particle arrays onto various substrates. The lattice spacing of the transferred monolayer colloidal crystal can easily be adjusted by thermally treating the PVA-silica spheres composite film for varied durations. We also have demonstrated the templating fabrication of periodic arrays of gold nanodots using a transferred monolayer nonclose-packed colloidal crystal as a structural template. The resultant plasmonic array exhibits high surface-enhanced Raman scattering enhancement factor (~3.8 × 10(7)) for adsorbed benzenethiol molecules.

Effective Collection and Detection of Airborne Species Using SERS‐Based Detection and Localized Electrodynamic Precipitation

Three different delivery concepts (standard diffusion, global electrodynamic precipitation, and localized nanolens-based precipitation) and three different SERS enhancement layers (a silver film, a nanolens-based localized silver nanoparticle film, and the standard AgFON) are compared. The nanolens concept is applied to increase the SERS signal: a factor of 633, when compared to a standard mechanism of diffusion, is observed.

Large surface-enhanced Raman scattering from self-assembled gold nanosphere monolayers

We demonstrate an average surface-enhanced Raman scattering enhancement on the order of 108 from benzenethiol molecules using self-assembled, macroscopic, and tunable gold nanosphere monolayers on non-templated substrates. The self-assembly of the nanosphere monolayers uses a simple and efficient technique that allows for the creation of a high-density, chemically functionalized gold nanosphere monolayers with enhancement factors comparable to those produced using top-down fabrication techniques. These films may provide an approach for the future development of portable chemical/biological sensors.

Plasmonic Amplifiers: Engineering Giant Light Enhancements by Tuning Resonances in Multiscale Plasmonic Nanostructures

The unique ability of plasmonic nanostructures to guide, enhance, and manipulate subwavelength light offers multiple novel applications in chemical and biological sensing, imaging, and photonic microcircuitry. Here the reproducible, giant light amplification in multiscale plasmonic structures is demonstrated. These structures combine strongly coupled components of different dimensions and topologies that resonate at the same optical frequency. A light amplifier is constructed using a silver mirror carrying light-enhancing surface plasmons, dielectric gratings forming distributed Bragg cavities on top of the mirror, and gold nanoparticle arrays self-assembled into the grating grooves. By tuning the resonances of the individual components to the same frequency, multiple enhancement of the light intensity in the nanometer gaps between the particles is achieved. Using a monolayer of benzenethiol molecules on this structure, an average SERS enhancement factor <EF> ∼10⁸ is obtained, and the maximum enhancement in the interparticle hot-spots is ∼3 × 10¹⁰, in good agreement with FDTD calculations. The high enhancement factor, large density of well-ordered hot-spots, and good fidelity of the SERS signal make this design a promising platform for quantitative SERS sensing, optical detection, efficient solid state lighting, advanced photovoltaics, and other emerging photonic applications.

Increase of SERS Signal upon Heating or Exposure to a High-Intensity Laser Field: Benzenethiol on an AgFON Substrate

The surface-enhanced Raman scattering (SERS) signal from an AgFON plasmonic substrate, recoated with benzenethiol, was observed to increase by about 100% upon heating for 3.5 min at 100C and 1.5 min at 125C. The signal intensity was found to increase further by about 80% upon a 10 sec exposure to a high-intensity (3.2 kW/cm^2) 785-nm cw laser, corresponding to 40 mW in a 40+/-5-um diameter spot. The observed increase in the SERS signal may be understood by considering the presence of benzenethiol molecules in an intermediate or 'precursor' state in addition to conventionally ordered molecules forming a self-assembled monolayer. The increase in the SERS signal arises from the conversion of the molecules in the precursor state to the chemisorbed state due to thermal and photo-thermal effects.

Controlled Synthesis of Ag/Ag/C Hybrid Nanostructures and their Surface‐Enhanced Raman Scattering Properties

SERS them right: Ag/C nanoparticles (see figure; red line) and Ag/Ag/C nanoclusters (blue line) were synthesized as platforms for surface-enhanced Raman scattering (SERS). The results showed that the Ag/Ag/C nanoclusters led to great enhancement of the Raman signals for methyl benzenethiol molecules.

Patterned nanoporous gold as an effective SERS template

We demonstrate large area two-dimensional arrays of patterned nanoporous gold for use aseasy-to-fabricate, cost-effective, and stable surface enhanced Raman scattering (SERS)templates. Using a simple one-step direct imprinting process, subwavelength nanoporousgold (NPG) gratings are defined by densifying appropriate regions of a NPG film. Both thedensified NPG and the two-dimensional grating pattern are shown to contribute to the SERSenhancement. The resulting substrates exhibit uniform SERS enhancement factors of at least107 for a monolayer of adsorbed benzenethiol molecules.

Self‐Assembled Large Au Nanoparticle Arrays with Regular Hot Spots for SERS

The cost-effective self-assembly of 80 nm Au nanoparticles (NPs) into large-domain, hexagonally close-packed arrays for high-sensitivity and high-fidelity surface-enhanced Raman spectroscopy (SERS) is demonstrated. These arrays exhibit specific optical resonances due to strong interparticle coupling, which are well reproduced by finite-difference time-domain (FDTD) simulations. The gaps between NPs form a regular lattice of hot spots that enable a large amplification of both photoluminescence and Raman signals. At smaller wavelengths the hot spots are extended away from the minimum-gap positions, which allows SERS of larger analytes that do not fit into small gaps. Using CdSe quantum dots (QDs) a 3-5 times larger photoluminescence enhancement than previously reported is experimentally demonstrated and an unambiguous estimate of the electromagnetic SERS enhancement factor of ≈10(4) is obtained by direct scanning electron microscopy imaging of QDs responsible for the Raman signal. Much stronger enhancement of ≈10(8) is obtained at larger wavelengths for benzenethiol molecules penetrating the NP gaps.