v2i2-pan

Research Paper

A Single Molecule Surface-Enhanced Raman Spectroscopic Study of Regioregular Poly (3-Hexylthiophene-2, 5-Diyl) on Nanostructured Silver Substrate

Daniel A. Clayton, Karson S. Brooks, and Shanlin Pan*

Department of Chemistry and Biochemistry, The University of Alabama, Tuscaloosa, Alabama 35487, USA. *Corresponding author, Shanlin Pan, email: span1@bama.ua.edu

Received 15 April 2018, revised 25 July 2018, accepted 26 July 2018

Publication Date (Web): July 26, 2018

© Frontiers in Science, Technology, Engineering and Mathematics

Abstract

This study is intended to increase the fundamental knowledge of structural and conformational dependence of single molecule Raman scattering characteristics of one of the most common organic semiconductors, regioregular poly (3-hexylthiophene-2, 5-diyl) (P3HT). Single molecule Raman spectra of P3HT are found to have extremely long lifetimes in comparison to Rhodamine 6G (R6G) single molecule trajectories due to the multiple chromophoric nature of the conjugate polymer molecules. Single molecule spectra evolution and dynamic changes in its trajectories are recorded for polymer chains on Surface-Enhanced-Raman-Spectroscopy (SERS) active silver substrate. Studies of the effects of four solvents, including chlorobenzene, dichloromethane, toluene, and tetrahydrofuran (THF), used to dissolve the polymer to prepare single molecules on the nanotextured silver surface show good agreement with the ensemble measurements. The SERS spectrum of single P3HT in toluene shows the most Raman bands, indicating that the solvent plays a critical role in defining the folding of the molecule on the surface. Time-dependent Raman spectra show single P3HT polymer chains have very stable SERS spectra in comparison with R6G molecules. An incident light polarization effects study indicates only a weak correlation of incident light polarization angle with plasmon enhanced photoluminescence of the silver nanostructure, and low sensitivity of single-molecule SERS to incident light polarization because the molecules are excited along multiple different axes due to their poorly defined shapes on the nanotextured silver surface.

Keywords

Single molecule, Surface plasmon, Nanoparticles, Blinking, Surface-enhanced Raman

Introduction

With the discovery of Surface Enhanced Raman Spectroscopy (SERS) by Jeanmarie and Van Duyne (Jeanmarie and Van Duyne RP 1977) as well as Albrecht and Creighton (Albrecht and Creighton 1977) in 1977, Raman spectroscopy has become a useful analytical tool for the development of ultrasensitive biosensors (Bizzarri and Cannistraro 2007b; Cao et al 2003; Culina et al 2003), and helping understand the nature of kinds of interface problems such as charge transfer (Milani et al 2011; Wang et al 2011; Van Hal et al 1999) and conformation of a molecule at a solid surface (Li et al 2012; Podstawka 2008; Sun et al 2008; Sambur et al 2012). A major breakthrough in the area, specifically by Nie and Emory (Nie and Emory 1997) and Kneipp and coworkers in 1997 (Kneipp et al 1997), was able to reveal that strongly coupled silver nanoparticles under resonance conditions can enhance the local field intensity to a point where Single Molecule Surface Enhanced Raman Spectroscopy (SM-SERS) was possible. Yet most of the molecules studied have been small organic molecules, such as Rhodamine 6G (R6G) (Michaels and Jiang 2000; Dieringer et al 2009; Constantino et al 2001; Kleinman et al 2001). Only a few reports have shown that this technique can be applied to larger molecules like polymers (Wang and Rothberg; Yu et al; Hu et al 2000; Huser et al 2000; White et al 2001; Schindler and Lupton 2005; Lee et 2006; Walker MJ et al 2007) and proteins (Habuchi 2003; Bjerneld 2002).

Conjugate polymers are a class of polymer molecules that have attractive optical and electrical properties for helping to understand the underlying photophysical properties of organic semiconductors and real device applications such as organic solar cells (Cowan 2012), light emitting diodes (Kulkarni 2004), and transistors (Yang 2005). Past studies on conjugate polymers and their thin films have indicated that the processing conditions lead to variability (Schwartz 2003; Peng 2005) in the morphology that can affect their photophysics and device performance (Collison et al 2001; Spano 2005). Recent studies have shown that SM-SERS is a powerful tool to help reveal the conformation change and charge transfer of a conjugate polymer at a surface (Wang and Rothberg 2007; Schindler and Lupton 2005).

The present study intends to show the validity of using SM-SERS as a way of investigating polymer dyes, specifically regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), a common dye used in solar cells (Li et al 2012). This molecule can have various conformations in different solvents and its conformation changes on a solid surface have yet to have been studied at the single molecule level (Brinkmann 2011). Further, the study intends to demonstrate the differences that can be gleaned from varying the solvent and incident field polarization angle.

Materials and Methods

Chemicals

HPLC grade toluene, HPLC grade tetrahydrofuran (THF), silver nitrate, potassium hydroxide, strong ammonia solution, nitric acid and HPLC grade isopropanol were used as received from Fisher Chemicals (Pittsburgh). D-(+)-dextrose, regioregular poly (3-hexylthiophene-2, 5-diyl) (MW 20,000-70,000, American Dye Source, Inc. Quebec, Canada) and HPLC grade chlorobenzene were used as received from Sigma-Aldrich (St. Louis, MO). Rhodamine 6G was obtained from Enzo Life Sciences (Farmingdale, NY) and used as received.

Slide Cleaning

Cover glass (No. 1, 22 mm2) was obtained from Corning Inc. (New York). A standard cleaning procedure was performed by first placing the glass slides individually in a glass staining jar and filling the jar with 10% w/w potassium hydroxide in isopropanol. The jar was capped and sonicated for 10 minutes. The potassium hydroxide solution was removed and then the process was repeated with deionized water then isopropanol and then deionized water again. The slides were then dried with nitrogen. Finally, the slides were placed in a UV/Ozone Pro cleaner (BioForce Nanosciences, Ames, IA) and each side was exposed for 10 minutes.

SERS Surface Preparation

SM-SERS experiment of P3HT was performed on a nanostructured silver substrate prepared using the Tollens’ silver mirror reaction (Saito et al 2002). To prepare the two solutions of dextrose and silver (I) diammo ion Ag(NH3)2+ needed for the Tollens silver mirror reaction, 0.10 M AgNO3, 0.80 M KOH, and 0.25 M dextrose (C6H12O6) were prepared separately. To make the silver diamine, 30 mL of the 0.10 M AgNO3 solution was added to a 125 mL Erlenmeyer flask, and 15 mL of the 0.80 M KOH solution was sequentially added to the flask. A brown precipitate immediately resulted from the mixture of the two solutions. Concentrated (15 M) ammonia was then added drop-wise until the solution was clear and colorless. Both the solutions of dextrose and silver nitrate were stored in separate dark glass bottles and cooled until usage. Equal aliquots of the dextrose and Ag(NH3)2+ solutions at room temperature were placed in a disposable plastic vial and mixed thoroughly. This mixture was then pipetted onto a cleaned glass slide. After three minutes, the mixture was rinsed with water and ethanol and allowed to dry. The resulting slide was uniformly covered with silver and moderately brown in color. The slides were examined using a scanning electron microscope (JEOL 7000, Tokyo, Japan) to discern the composition of the slides. The absorbance of the silver was also measured using an ultraviolet-visible spectrophotometer (Cary 50, Santa Clara, CA).

Sample Preparation

Single molecule Raman performance of P3HT on nanostructured silver surface was compared with that of Rhodamine 6G. For the ensemble measurements, a 1_10-5 M solution of R6G in water and 1_10-5 M (monomer concentration) solutions of P3HT in each of toluene, THF, dichlorobenzene, and dichloromethane were prepared. Each one was independently spin-coated onto a Tollens’ silver mirror coated glass slide at 2,000 rpm for 60 seconds. Single P3HT molecules were probed after coating from different solvents to compare with R6G. For the single molecule measurements, a 1_10-9 M solution of R6G in water and 1_10-9 M (monomer concentration) solutions of P3HT in each of toluene, THF, dichlorobenzene, and dichloromethane were prepared and spin coated in the same way.

Instrumentation

Raman signals were obtained using a homemade confocal microscope set-up. A Nano-View 200-2/M nanopositioner (Mad City Laboratories, Madison, WI) was mounted to the top of an Olympus IX-71 Inverted Microscope (Olympus Corporation, Center Valley, PA) for scanning. The sample was excited using a 633 nm self-contained HeNe laser (Thor Labs, Newton, NJ) at ~9.5 uW and imaged with a 100x numerical aperture oil immersion objective (NA 1.3). The emission was collected with the same objective and passed through a 633nm laser filter set with long pass emission (Z488LP, Chroma Technology, Brattleboro, VT) and a 633 nm Notch Filter (Edmund Optics Inc.). The photoluminescence signal was then split, sending one part into a spectrometer with a liquid-nitrogen-cooled digital CCD spectroscopy system (Acton Spec-10:100B, Princeton Instruments, Trenton, NJ) through a monochromator (Acton SP-2558, Princeton Instruments, Trenton, NJ) and the other to an avalanche photodiode (APD, SPEM-QAR-15, PerkinElmer). A PC 6602 card from National Instruments Inc. was used for data acquisition. Data collection and control of the photon counter and nanopositioner were done using LabVIEW 8.5 (National Instruments Inc.). Data analysis of the scanned images was done using MatLab (MathWorks, Natick, MA). The intensity trajectories and Raman spectra were analyzed using Origin 6.1 (OriginLab, Northampton, MA).

Results and Discussion

The resulting nanotextured SERS slide is uniformly covered with silver and moderately brown in color. The slides were examined using a scanning electron microscope to discern the silver morphology (Figure 1A). The absorbance of the silver (Figure 1B) shows the optical density of the SERS substrate is around 0.18 at its plasmon absorption maxima of 445 nm. We found this would be the optimal silver coverage and resonance condition for our single molecule study. To confirm the viability of the Tollens silver mirror reaction substrate for SM-SERS, we first examined the background SERS signal of the bare silver substrate. Many “hot spots” were located on the sample emitting primarily silver fluorescence, but two Raman peaks at around 1258 and 1344 cm-1 were present. These peaks were attributed to nitric acid not being fully removed by the washing step (Grothe et al 2006; Lucas and Petitet 1999). It should be noted that previous experiments have reported various contaminants (Borys and Lupton 2011), such as atmospheric carbon, being present on single molecule substrates indicating that such contaminants are fairly common. R6G was used as the probe for confirming single molecule capability. As shown in Figure 2 A and B, we recorded images of light scattering of the SERS substrate containing many hotspots but only a few spots were found to contain single molecule Raman signatures. As can be seen in Figure 2C, Raman spectrum of a single “hot spot” was taken using the confocal microscopes scanning capabilities to compare with a spectrum of the ensemble. Literature (Moore et al 2005; Bizzarri and Cannistraro 2007a; Walter et al 2008) confirms the correlation between “hot spots” and the luminescence/Raman intensity enhancement of a single R6G molecule. The Raman spectra signature is consistent with the SERS spectra of ensemble R6G molecule although some Raman peaks disappear from the single molecule spectra because the single molecule Raman is sensitive to the molecular orientation and interaction with SERS substrate, yield specific Raman modes been greatly enhanced by the silver substrate. We compare the ensemble SERS spectra of P3HT and single molecule P3HT from THF in Figure 2D; similar consistency can be observed for ensemble P3HT and single polymer chain SERS spectra. The C=C symmetric ring stretch of P3HT at 1459 cm-1 is reported in the literature (Bazzaoui 1995) as being the most intense band, and appears in all measurements, though slightly blue shifted for SERS spectra. The blue shift might be due to the partial oxidation state of the polymer chain upon the photo-induced charge transfer at a hotspot and is consistent with the previous report (Bazzaoui 1995). Also, the C=C anti-symmetric ring stretch normally located at 1498 cm-1appears to be absent. This band, which is normally used to help determine the conjugation length of the polymer, is thought to be present but difficult to pick out from the broad symmetric ring stretch when combined with the background fluorescence. It should be noted that both Figure 2 C and D include no fluorescence background from the silver substrate. We made corrections to the SERS spectra by subtracting the fluorescence signature from the original spectra.

Figure 1. SEM image (A) of nanotextured silver surface for single molecule SERS and its absorption spectrum (B). 

The time evolution of selected single molecule R6G spectra in Figure 3A shows a high rate of intensity fluctuation indicative of single molecule behavior (Bizzarri and Cannistraro 2007b; Weiss and Haran 2001) as well as the possible movement of the molecule. Differences in the peak intensity can be accounted for through variance in the angle of scattering, molecular orientation changes, and differences in the excited molecular motion (Wang and Rothberg 2005). The identifying peaks at 1392 and 1541 cm-1 indicate the presence of the R6G (Guthmuller and Champagne 2008; Jensen and Schatz 2006; Watanabe et al 2005). In comparing the various acquisitions in a single molecule of R6G over 4 minutes (Figure 3B), the intensity of the acquisition is first moderate for the first acquisition; however, after the first acquisition, the intensity increases significantly. This remains relatively stable throughout many acquisitions, decreasing only slightly with each subsequent acquisition. However, after 150 s, the intensity decreases significantly, and the intensity further decreases at 210 s and remains low for the remainder of the collection period. On the other hand, the spectra of single P3HT molecules from chlorobenzene show that the molecule’s Raman signal is much more stable throughout many acquisitions. As seen in Figure 3B, the relative intensity remained increasingly stable throughout the eight 30 s acquisitions. This lack of blinking in the molecule can also be seen in the molecule’s trajectory, which shows a stable slightly downward sloping line. The trajectory does show several intense, short-lived peaks; however, these peaks are due to the minute variations of the orientation of the molecule or charge transfers occurring in the sample. Unlike in R6G, these events in P3HT do not affect the overall stability of the molecule, and, therefore, P3HT is much more stable than R6G because of the conjugate polymer structure of P3HT containing many repeating monomers. Additionally, the hydrophobic side chain may help minimize the photo-oxidation of the polymer backbone in air further increasing the stability.

Figure 2. A and B are scanning Raman image of Rhodamine 6G (R6G) and P3HT, respectively, on the Tollens silver mirror substrate with multiple hotspots; C is the comparison of SERS spectra of single R6G molecule and ensemble R6G molecules, and D is the SERS spectra of single P3HT molecule and ensemble P3HT molecules.

Figure 4A shows the single molecule Raman spectra of the P3HT cast from four different solvents onto the SERS active silver substrate. The C=C symmetric ring stretch of P3HT around 1459 cm-1 appears for all solvents while dramatic differences for other Raman peaks exist. We think this is due to the conformation difference for P3HT when the molecules memorize the conformation in solution when they are transferred onto a nanotextured silver surface as shown by similar conformation memory effect of single polymer chains of MEH-PPV, Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] on a solid substrate (Rothberg et al 2000). Further studies are needed to confirm the conformation differences for all these solvents. For dichloromethane, the SM-SERS peaks match well with the ensemble confirming the identity of the P3HT. Importantly peaks such as the C-S-C bond at 683 cm-1 and the C-H bond at 1210 cm-1 are markedly absent in almost all the spectra, a common phenomenon in single-molecule Raman studies where particular bands apparent in the ensemble are absent in the single molecule. Other peaks like the C=C bond at 1521 cm-1 are present in the shown spectra but absent in other measured spectra, indicating not only SM behavior but also implying that the orientation along the varied silver surface is not uniform when casting the single molecule. This should further indicate that local variability will occur with variances in the polarization angle of the incident light, with that aspect being further probed below. The peak at ~1498 cm-1 is related to the actual backbone of the polymer and can be used to indicate the conjugation of the polymer chain. In past cases, lower intensities and sharper peaks in this region have indicated more highly conjugated polymers with less overall flexibility along the backbone. Further, the intensity ratio of the 1459 cm-1 over the 1498 cm-1 peak gives an inverse relationship with the conductivity of polythiophene films (Weiss and Haran 2001) and is a value worth noting in the single molecule systems. However, due to the high background fluorescence, it is extremely difficult to find the antisymmetric stretch band, without which the ratio cannot be calculated. The Raman signature of single P3HT molecule cast from the other three solvents (chlorobenzene, THF, and toluene) are shown in Figure 4 A. We found each single molecule run corresponds well with its ensemble measurements, showing some variance much as dichloromethane showed variance. Further, P3HT single molecules from each solvent demonstrate slightly different spectra. Raman Peaks at ~650 cm-1, which correspond closely with the distorted conformation around the inter-ring single bond also known as a kink in the polymer (Jakubiak et al 2000; Akimoto et al 1986), are present in toluene and THF spectra but not in single polymer chains cast from chlorobenzene and dichloromethane. This is thought to be due to a different polymer conformation containing no or fewer kinks in a more polar chlorine-containing solvents in comparison to THF and toluene.  In addition, dichloromethane does not exhibit a peak at 997 cm-1 associated to C-C Raman mode between the ring and alkyl chain. This is believed to be due to solvent memory influencing the side chains folding parameters upon the silver surface causing them to preferentially lay perpendicular with the surface and thus not be enhanced by the silver surface plasmon. A similar solvent dependent conformation change of P3HT on a solid substrate such as TiO2 has been observed previously for the ensemble system (Kwong et al 2004). Good solvents such as xylene and chlorobenzene would lead to extended polymer chains which have sufficient contact with the substrate causing improved exciton dissociation and short circuit current for a solar cell.

Figure 3. Raman spectra and time flow of the spectra of single molecule R6G (A) and P3HT (B).

The single molecule trajectories of P3HT were compared with the ones of R6G as shown in Figure 4 B. The intensity trajectories of P3HT from all kinds of solvent are relatively stable in comparison to R6G.  

Figure 4. Single molecule spectra P3HT coated from chlorobenzene, dichloromethane, toluene, and tetrahydrofuran (A), and corresponding typical single molecule Raman trajectories of P3HT from these four different solvent in comparison to single molecule R6G and SERS substrate (B).

Normally this would indicate several molecules being present with blinking behavior indicating a single molecule. However, previous studies (Walter et al 2007) done by SERS with polymers have suggested that the polymer can be thought of as having multiple specific chromophores. Other studies have indicated that it is possible to have both a system of non-isolated chromophores that exhibit energy transfer to the lowest energy level chromophore (Yu et al; Hu et al 2000; Lee et al 2006) and isolated chromophores along the same polymer chain that fails to transfer energy (Huser et al 2000). The ability to energy transfer between these chromophores causes distinct broadening of the photoluminescence background that is overlaid on the Raman signal. The stair step nature of the intensity trajectory indicates that multiple chromophores are likely exhibiting Raman signals indicating a low level of intra-molecular conjugation. The majority of intensity trajectories for P3HT molecules from each solvent indicate similar low amounts of conjugation to that seen in dichloromethane. However, not all intensity trajectories showed the normal stair step decrease in intensity and thereby indicate the possibility that high levels of intra-molecular conjugation have occurred in some single molecule experiments. This appears to be largely solvent independent within the observed solvents and may be more a function of the local surface. This is dramatically different from single molecule trajectories of R6G that show rapid stepwise changes in the trajectories due to changes in the molecular orientation and/or local heating before becoming photobleached. We found no such behavior for bare SERS substrate, except very short bursts in fluorescence background from small silver clusters that emit strong luminescence which can undergo dynamic changes because of the local photochemical reaction of silver oxide and silver as shown previously (Clayton et al 2010).

The trajectories shown in Figure 4 include both Raman and the silver background fluorescence. To further demonstrate the solvent dependence and dynamic changing in the local field-enhanced Raman spectra, time evolution Raman spectra are shown in Figure 5. Consistent with the results of trajectories in Figure 4, Raman spectra of P3HT from all kinds of solvent are more stable in comparison to single molecule R6G. The appearance of particular Raman bands also provides strong proof of actual single molecule activity, since it is a common occurrence for some bands not visible in the ensemble to be visible in the SM spectra. For example, Raman bands in THF at 724 cm-1 of the C-S-C band appear initially but disappear over time, indicating that the local area is likely undergoing heating and thus the SM is undergoing conformational shifts over time (Wang and Rothberg 2007), with it being unlikely but possible that the silver surface is changing through photoreduction (Bazzaoui et al 1995). These findings hold for more than 20 molecules probed.

Figure 5. The time evolution Raman spectra of a single molecule of P3HT coated from (A) Toluene, THF (B), dichloromethane (C), and chlorobenzene (D).

Stable single polymer Raman spectra would allow us to study the polarization dependence of the Raman intensity of a molecule. To fully confirm the single molecule nature, a polarization study was performed using P3HT, since the SERS enhancement is dependent upon the polarization of the incident light and its interactions with the orientation of the molecule and the local plasmonic field (Furukawa et al 1987; Lyon et al 1998; Yoon et al 2008). To accomplish this, a half plate was placed in the laser line attached to an electric motor with known acceleration and velocity. The plate was rotated from 0º to 90º then back 0º with the cycle being repeated three times. This alters the polarization angle of the incident light so that its effects may be tracked. One of the major difficulties when performing this sort of experiment is that the dichroic mirror used in the microscope will not reflect all polarization angles equally. Therefore, the first experiment was performed with a thin film of P3HT with no silver as shown in Figure 6 A to correct the actual light intensity in the laser focal volume. This leads to a situation where the P3HT is homogenous on the glass, and no differences in local geometry of the substrate should affect the overall output light. Figure 6 A shows the sinusoidal intensity traces taken of the P3HT thin film while the polarization angle is changed. The fluctuations in intensity are fairly uniform and despite some photo-bleaching due to the photoluminescence signal being largely fluorescence, lead us to believe that the dichroic mirror is having a standardized effect on the power of the incident light at the sample. The laser intensity modulation by the dichroic is about 10% of the total laser intensity. With the nature of the dichroic’s effect on the light intensity determined, it is now important to see if a uniform effect would occur on a nanostructured silver substrate. As such, Figure 6 A shows five trajectories of selected hotspots on silver substrate in comparison to a P3HT film. The usual blinking phenomenon seen in silver is present and overlaid on a stronger background intensity modulation. The blinking phenomenon arises from the stochastic bursts of photoluminescence caused by the aggregation and formation of photoactive silver nanostructures (Clayton et al 2010). These structures are extremely small, a few silver atoms in size, and thus produce a stochastic effect much like that of a set of single molecules. With the experiment being performed under air, one also expects that oxidation of the silver structures can cause bleaching, while the photoreduction occurs at other points causing the reappearance of the silver photoluminescence. This modulation appears in all samples but is not uniform across the film in either time or intensity from spot to spot as shown in Figure 6A. The lack of uniformity in the modulation for some of the hotspots indicates that local field enhancement at some hotspots of the nanostructured silver substrate is strongly dependent on the geometry of the local silver nanostructure instead of the incident light intensity. Such local structural dependence of the surface enhancement is critical for single molecule Raman enhancement. Other spots we have probed show a strong correlation between the background fluorescence of silver and incident light intensity modulation. This is not unexpected since the silver has been reported as enhancing the intensity of several orders of magnitude due to the local field effect. Further, the difference in intensity spectrums is further supported by the silver mirror process which leaves a poorly defined system. Thus, one would expect to see multiple different intensity spectrums dependent upon the poorly defined, but strongly enhancing local field.

Figure 6. (A) Photoluminescence intensity trajectories of several selected hotspots (1-5) on silver substrate in comparison to a P3HT thin film at various laser polarization angle; (B) Single molecule P3HT intensity trajectory shows definite modulation due to the differences in laser intensity. Fluorescence modulation on ensemble P3HT intensity trajectory is shown in each figure to show changes in the laser intensity on the sample surface.

Finally single molecule P3HT samples were analyzed while the polarization angle of the incident laser is changed as shown in Figure 6 B. Overall, no large amount of uniformity is seen in the trajectory of single-molecule Raman signal upon the change of polarization angle of the incident beam as the total collected light scattering signal of P3HT is overwhelmed by the background fluorescence signal of silver substrate as shown in Figure 3B and Figure 6A Most P3HT molecules we have spotted on the nanostructured silver surface show that their light scattering seems to consistently follow the trend of the laser intensity as we change the polarization angle with local field effects appearing to have no effects. Because the silver substrate is poorly defined it cannot be ruled out that the local field may primarily be in resonance along the maximum polarization of the incident laser but is thought to be unlikely to continuously occur. Thus, the polymer chains themselves are not laid out flat along the surface and instead are folded in many different directions. This leads to chromophores being excited along multiple different axes, and not just when the substrate’s local field and incident laser are primarily interacting, and thus seeing the laser intensity modulation playing a large role in the excitement of the molecule. In addition, the polymer size is much smaller than the geometric size of the silver nanoparticles in the focus volume, therefore, the overall SERS of the molecule has to follow the field intensity of the nanostructure.

Conclusions

To conclude, this study successfully demonstrates single molecule characteristics of P3HT as a representative polymer dye commonly found in dye-sensitized solar cells. Our results show that the multiple chromophores allow a higher stability and less overall blinking, but time-resolved spectra do indicate differences in the excited Raman bands over time in comparison to single molecule Raman of R6G on a nanostructured silver surface. Further work indicated that solvent effects are apparent with particular bands being absent, and this data may possibly provide better insight into the folding structure of the molecule dependent upon its solvent memory. Finally, the photoluminescence background of the nanostructured silver substrate and single-molecule Raman scattering show only weak polarization dependence. The collection of the polarization data is poor due to the undefined nature of the substrate causing a strong photoluminescence background. However, it was noted that it is likely that the polymer does form in a folded pattern with various chromophores showing preferential polarization on the nanostructured silver surface.

Acknowledgements

This research work was supported by the Department of Energy under Award Number (s) DE-SC0005392. SP thanks the financial support from the national science foundation (NSF award CHE 1508192 and OIA-1539035) to support the final stage of this work. We also thank Central Analytical Facility (CAF) at The University of Alabama for providing TEM instrument for this study. The technical support from Mr. Johnny Goodwin during TEM analysis is greatly appreciated.

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Citation:

Daniel A. Clayton, Karson S. Brooks, and Shanlin Pan* (2018) A Single Molecule Surface-Enhanced Raman Spectroscopic Study of Regioregular Poly (3-Hexylthiophene-2, 5-Diyl) on Nanostructured Silver Substrate, Frontiers in Science, Technology, Engineering and Mathematics, Volume 2, Issue 2, 92-104