Conducting Polymers Increases PIERS Spectroscopic Signals Fivefold

Semiconductors and plasmonic nanomaterials can be combined to develop photocatalytic and detection systems. Nanoscale composites containing semiconductors and metals can improve plasmon-assisted spectroscopy and improve catalytic performance by modulating the charge states of metals.

Study: Hybrid composites based on conducting polymers and plasmonic nanomaterials applied to catalysis and sensing. Image Credit: GiroScience/Shutterstock.com

An article published in Express Research Material demonstrated improved Raman scattering by using a conductive polymer as a semiconductor platform.

Introduction to Raman Spectroscopy

Vibration spectroscopy is a method for determining the molecular skeleton caused by vibrational vibrations of particles. Raman scattering is a popular vibration spectroscopy technique.

Raman signals produce different spectra for molecules and are useful for analytical purposes. However, Raman scattering has drawbacks and requires considerable improvement before its widespread use in analytical studies can be realized.

In the spectroscopic technique, only one out of every 106 photons is converted into Stokes Raman scattering, resulting in insufficient analytical signal strength. The introduction of nanostructures can enhance the Raman effect using plasmonic amplification, enabling Raman sensing for single molecules.

Nanostructured materials can be fabricated using advanced material characterization and processing techniques, including metallic nanostructures with various morphologies and characteristics.

fiImage 1. (a) FTIR spectrum of P3HT treated and untreated: PCBM also showed temperature differences of 50, 100, 150, 200, and 250 °C in the oven for 40 minutes (b) fluorescence spectrum of P3HT: PCBM before and after thermal annealing at 250 °C. (c) The water contact angle of the P3HT mixture: PCBM shows different temperatures from 0 to 250 °C. (d) ) SEM images of (i) Ag NPs of 40 nm size (ii) P3HT. (iii) PCBM. (iv) P3HT NP: PCBM/Ag.

Increase Raman Scatter

A significantly amplified electromagnetic (EM) field is generated on the surface of metal nanostructures by local surface plasmon excitation. Chemical processes or EM techniques can enhance the effects of Raman.

Chemical processes have received less attention due to lower Raman amplification effects compared to those from EM. Charge-transfer activities in molecular-platform complexes, such as those caused by derivative resonance coupling, enhance the Raman effect signal.

Photo-Induced Enhancement Raman Spectroscopy

It is possible to achieve surface-enhanced Raman scattering (SERS) by incorporating photo-triggered oxygen vacancies on the surface of semiconductor materials such as TiO.₂.

The oxygen vacancy state is shown to promote derivative coupling among semiconductor materials, defects, metals, and analytes. This clutch, in turn, enhances Raman’s effect. This mechanism is dubbed photo-induced enhanced Raman Spectroscopy, or PIERS, for short.

PIERS can be used to detect small amounts of different small molecular analytes for various semiconductor materials. Along with the PIERS method, the semiconductor material can establish a junction between the metal and itself, enabling efficient separation of carriers via the Schottky junction.

Schottky junctions are generated when semiconductor materials and metals are in close proximity, and charge carriers cross from one part to another, which helps bring their Fermi levels into balance.

Figure 2. SERS spectra of 4-nitrophenol (4NP) recorded on P3HT:PCBM (a) SERS were recorded on polymer blends annealed at different temperatures. (b) SERS intensity normalized before (black) after heat treatment (red). (c) Schematic of the oxidation reaction and formation of 4-aminophenol from 4-Nitrophenol.

Which semiconductor is used in this study?

The combination of n-type and p-type semiconductor materials, such as P3HT:PCBM, is a commonly used conductive organic semiconductor.

The P3HT polymer exhibits a large charge movement in a highly crystallized sheet and functions as an electron donor in the photo-excitation phase. Exciton breakdown is made possible by complementary PCBMs. When used with gold (Au) or silver (Ag) nanoparticles, the P3HT:PCBM polymer combination creates a Schottky junction.

Research methodology

As a framework for enhancing the Raman effect, P3HT:PCBM and Ag nanoparticles were mixed in this work. The team demonstrated that heat treatment of this plasmonic-semiconductor complex significantly enhances the surface-enhanced Raman spectroscopic signal that the analyte can generate. In addition, the team demonstrated that the polymer complex supports plasmonic catalytic processes.

The team also examined the photoluminescence (PL) spectrum of the polymer complex. They measured the contact angles for different specimens before and after heat treatment.

Figure 3. Energy band diagram illustrating the electronic transition between P3HT:PCBM and NP Ag and MB analyte molecules. The red line shows the electron transitions excited by the Raman excitation laser.

Key Findings from the Study

In this study, the team demonstrated how the intensity of the PIERS signal was increased approximately fivefold through the use of a conductive polymer material having plasmonic properties. A support mechanism based on charge transfer was found to promote the oxidation of the desired molecule on the active plasmonic nanostructure.

Heat treatment of the polymer blend enhances the impact of self-entrapped local stimuli on photoluminescence.

Using an optimized chemical mechanism, a charge transfer based approach enhances the Raman effect signal. This study demonstrates how conductive polymers can be used as semiconductor frameworks for plasmonic catalysis and detection.

Reference

Alanazi, AT, & Rice, JH (2022). Hybrid composites based on conducting polymers and plasmonic nanomaterials applied to catalysis and sensing. Express Research Material. Available at: https://iopscience.iop.org/article/10.1088/2053-1591/ac7d9a

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