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Beyond the diraction limit: scanning near-eld optical microscopy
All the optical microscopies and spectroscopies previously described can provide multiple and complementary information about the nature, the structure and the properties of materials or molecules as well as their transformation under electrochemical potential control for example. Today, the strong trend towards nanoscale science and technologies has catalyzed the development of new fabrication, manipulation and investigation tools able to reach this scale. Electron microscopies are attractive by providing subatomic details but their implementation in situ is di- cult. Moreover, the beam-induced damages to the sample may alter the mechanism under study. Another powerful approach is based on scanning probe microscopies (SPM) such as STM, AFM and their implementation in combination with optical methods. A new paradigm has emerged out of these developments: scaling down the matter investigation towards the nanoscale has revealed news borders beyond which new physical phenomena and eects were observed and even become prominent.
Particularly, in optics, a new domain called nano-optics has emerged and aims at understanding optical phenomena that occur below the diraction limit. Indeed, classically, objects observed with optical spectroscopies cannot be distinguished if they are separated by a distance smaller than roughly half of the wavelength used for their observation (200 nm in the visible range).
Optical diraction limit
The optical diraction limit can be understood considering that the propagation of a photon in free space is determined by the dispersion relationship connecting its angular frequency ! and its wavevector k = p k2x + k2 y + k2 z via the light velocity c : ~! = c ~k (1.23).
Considering Heisenberg’s uncertainty relationship on the spatial position and the momentum p of a particle in a particular direction: x px ~ 2 (1.24) we can write for a photon the relationship between the spatial connement and the spreading in magnitude of the wavevector in one particular direction: x 1 2kx.
Principles of near-eld optics
An approach for improving spatial resolution in optical imaging is provided by Scanning Near-Field Optical Microscopy (SNOM) which allows the optical imaging of features below the diraction limit near a surface. SNOM is usually coupled to a scanning probe microscope (SPM) allowing a topographic and optical imaging of the samples combining both the single digit nanometre scale of the SPM and the fast dynamics of optical measurements.
The optical near-eld can be dened as the non-propagating electromagnetic eld surrounding an object illuminated with a propagating electromagnetic eld (light, called the optical far-eld ). The optical near-eld exists in a restricted region of space (generally less than 100 nm) around the irradiated object. As the optical near-eld cannot propagate it cannot be directly measured with a detector but requires back-coupling to far-eld through interactions with matter.
The rst idea of SNOM was originally proposed by Synge in a pioneer paper in 1928. [48] He suggested the illumination of a sub-wavelength aperture in an opaque lm placed at a sub-wavelength distance on the top of a transparent substrate to be imaged: the light passing through the apperture is focused on the sample and transmitted to a detector through a non-diraction-limited process. The implementation of this idea could be practically demonstrated with an electromagnetic radiation in 1972 by Ash and Nicholls (who actually ignored Synge’s work) by using a microwave radiation with wavelength of 3 cm passing through a hole with an aperture diameter of 1:5 mm: a resolution of a 500 m periodicity. [49] These rst experimental evidences opened the way to the domain of near-eld microscopy.
The rst experimental evidences of SNOM carried out with an excitation source in the visible range were obtained a decade later by Pohl et al. with a 20 (25 nm) resolution [50] and Lewis et al. with a 14 (50 nm) resolution. [51] Finally, one decade later, Betzig et al. acquired the rst single molecule uorescence imaging with a resolution. [52] All these experiments have been performed in a conguration similar to the one originally proposed by Synge. Other congurations have been described so that SNOM can be divided arbitrarily into three general classes, organized by the type of near-eld probe employed:
nano-aperture-based techniques called aperture SNOM (a-SNOM).
methods using sharp tips acting as optical transducers called scattering SNOM (s-SNOM).
and strategies based on advanced tip designs that involve sophisticated optical antenna and/or apertures, incorporating concepts from the eld of plasmonics.
In the following, each of these categories will be briey described with a few examples of applications in the analysis of solid-liquid interfaces.
Aperture SNOM
The most commonly used SNOM approach is the one imagined by Synge, the aperture SNOM (a-SNOM): it makes use of conventional aperture-based probes which are usually metal-coated tapered dielectric waveguides with a sub-wavelength aperture at the apex of the structure (see Figure 1.4a). The metal-dielectric interface at the apex of the tip allows a localized evanescent wave (near-eld) to leak out at the aperture creating a light nano-source in the vicinity of this aperture. It can also be used as a nano light collector (see Figure 1.4b). The size of this spot, and then the resolution is mainly dened by the size of the aperture (and not by the wavelength anymore). This approach enables background-free imaging because sample illumination occurs only within the nanoscale light spot created at the aperture.
Photothermal approach for nano-IR
A very dierent nano-IR system was developped by A. Dazzi in 2005 and commercialized by the Anasys company. It is based on the photothermal expansion of materials upon excitation by an IR laser as depicted in Figure 1.6. With this system a full local IR spectrum can be recovered by tuning the IR laser wavelength and by analyzing the cantilever response to the absorption-induced mechanical stress of the sample. It has found a wide range of applications from biology to plasmonics that are summarized in a review by Dazzi and Prater. [67] However, no nano-IR studies have been reported in liquids using this system due to strong absorption of water in the IR range.
Tip-enhanced Raman Spectroscopy: Raman be- yond the diraction limit
The successful combination of the Raman enhancement by SERS and of the nanoscale analysis by SNOM or SPM was achieved in 2000 simultaneously by the groups of Kawata [68], Pettinger [69], Anderson [70] and Zenobi [71] pioneering a new eld in enhanced Raman spectroscopies: tip-enhanced Raman spectroscopy (TERS).
TERS can be seen as nanometric single SERS hot spot positioned at the apex of a SPM probe enabling the chemical imaging of a sample. The strength of this approach is that the sample under scrutiny is not altered and its Raman response can be extracted at each point of the sample by a strong and localized electric eld (near-eld) by scanning the metal tip over the sample. A simple scheme on Figure 1.7 compares TERS to SERS and confocal Raman spectroscopy.
Pioneer experimental TERS evidences
The rst reported TERS experiments by the groups of Zenobi, Kawata and Anderson were carried out in an inverted conguration on transparent samples: the illumination of the tip and the collection of the TERS signal was achieved using an inverted microscope. In all studies the molecules were Raman resonant ( e.g. dyes) ensuring a strong and even detectable far-eld signal which enabled the quanti- cation of the enhancement due to the presence of the tip. Anderson also reported Raman enhancement from a sulfur lm deposited on a glass slide using a gold-coated AFM tip. [70] Zenobi et al. [71] used BCB (Brilliant Cresyl Blue) molecules deposited on a glass support and a silver-coated AFM tip. They could report an enhancement factor of 104. They also reported TERS signal from a C60 monolayer. Kawata et al. [68] reported TERS from a thin layer of Rhodamine 6G (Rh6G) deposited on thin silver plates on glass and silver-coated AFM tip. They reported an enhancement factor of 40. However, this enhancement factor is highly underestimated as the far-eld signal they observed was probably also originating from several SERS hot spots due to the presence of silver nanoplates in the focus of the laser.
The same year Pettinger’s group reported TERS from BCB deposited on a thin gold layer on glass with a silver STM probe. [69] BCB and Rh6G have been widely used in TERS experiments as standard reference molecules. They provide strong Raman signal and allowed TERS to reach a breakthrough in 2008 with the detection of single BCB molecules at very low laser power in UHV. These results were reporter by Pettinger’s group [73] with a 106 enhancement factor.
Enhancement mechanisms and gap mode conguration
In TERS a gold or silver probe tapered tip is brought at the vicinity of a surface, below ca. 1 nm, and a laser source is focused on its apex, the polarization being linear and aligned with the shaft of the tip. The laser induces a resonant excitation of the SP at the apex of the tip creating a strong and localized hot spot such as in SERS. As visible excitation is commonly used in Raman spectroscopy, only gold and silver tips can support SPR. The enhancement mechanism is parent to the SERS enhancement i.e. with an electromagnetic mechanism based on the excitation of the plasmon resonance and a chemical mechanism that add to the lightening rod eect described in the s-SNOM section.
In gap mode TERS a noble metal substrate is used as substrate for the analyte. This conguration is singular as the tip and the sample can couple optically and form a new antenna with dierent plasmonic properties. One can see this interaction by considering the charge accumulation at the tip surface as a point dipole: by reducing the gap, a mirror dipole is formed within the sample by coulombic repulsion so that the LSP of the probe and the SPP of the surface couple to produce an hybrid plasmonic mode. [75] When the tip-sample distance increases TER signal evolution exhibits a behavior very similar to the one observed in the gap between two NPs. [76] Pettinger et al. examined the TERS enhancement and the plasmonic properties of the tip by varying the gap distance. By varying the gap between 1 and 10 nm they could observe a strong and rapid decrease in TERS and background intensity which highlights the strong eect of the optical coupling in the gap mode. They also analyzed background signal (lorentzian shape) in TER spectra and assumed its maximum to be the LSPR position of the tip-sample plasmonic antenna. They could observe a blueshift as the distance increases similarly to what is observed between two NPs.
Another consequence of the gap-mode in TERS is the connement of the near- eld below the tip. Beck et al. calculated the eld enhancement and its extent in a gap between a 10 nm radius gold tip and a gold substrate. They found that as the gap is larger than the tip radius, the near-eld is not aected and the enhancement can be compared to the one of a free-standing tip (apex mode regime). However below this value the near-eld is increasingly conned (hybrid gap plasmon) to roughly one fourth of the initial connement. [75] This phenomenon is shown in Figure 1.9.
Table of contents :
Introduction
1 Raman spectroscopy at the nanoscale for electrochemistry: state of the art
1.1 Raman scattering and spectroscopy
1.1.1 Light-matter interactions in the UV-Vis and IR range
1.1.2 Theory of Raman scattering
1.1.3 Raman cross section
1.1.4 Enhancement of the Raman scattering
1.2 Fundamentals of plasmonics and surface-enhanced Raman scattering .
1.2.1 Surface Plasmons
1.2.1.1 Optical properties of noble metals
1.2.1.2 Localized Surface Plasmon Resonance
1.2.2 Surface-Enhanced Raman Spectroscopy
1.2.2.1 SERS enhancement mechanism
1.2.2.2 Electrochemical SERS
1.2.2.3 Shell-Isolated Nanoparticles Enhanced Raman Spectroscopy
1.3 Beyond the diraction limit: scanning near-eld optical microscopy .
1.3.1 Optical diraction limit
1.3.2 Principles of near-eld optics
1.3.3 Aperture SNOM
1.3.4 Scattering SNOM
1.3.5 Photothermal approach for nano-IR
1.4 Tip-enhanced Raman Spectroscopy: Raman beyond the diraction limit
1.4.1 Pioneer experimental TERS evidences
1.4.2 About TERS enhancement
1.4.3 Enhancement mechanisms and gap mode conguration
1.4.4 Spatial resolution in TERS
1.4.5 Feedback Mechanisms
1.4.5.1 AFM-TERS
1.4.5.2 STM-TERS
1.4.5.3 TF-TERS
1.4.6 Fabrication of AFM and STM-TERS tips
1.4.7 Other types of SNOM and TERS tips
1.5 Probing and inducing chemical reactions with TERS
1.5.1 Catalytic reactions monitored by TERS
1.5.2 Plasmonic tip-induced reactions
1.6 Electrochemical TERS
1.6.1 Congurations proposed for TERS in liquids
1.6.2 EC TERS
1.6.3 Challenges for EC-TERS
2 Description of TERS experiments
2.1 Description of the setup for microRaman and TERS experiments
2.2 TERS tips manufacturing
2.2.1 Fabrication of gold TERS tips
2.2.2 Fabrication of silver TERS tips
2.2.3 Prevention of TERS tips degradation
2.3 TF-TERS experiments
2.4 Conclusion
3 Tip-Enhanced Raman Spectroscopy imaging of opaque samples in organic liquids
3.1 Introduction
3.2 Description of the experimental setup
3.2.1 Optical coupling
3.2.2 Spectroscopic characteristization of the solvent and of the molecular layer
3.2.3 Focusing of the laser at the apex
3.2.3.1 TER in the air
3.2.3.2 TERS in hexadecane
3.3 TERS imaging in organic liquid
3.4 Conclusion
4 Electrochemical Tip Surface-Enhanced Raman Spectroscopy
4.1 Context
4.2 Description of the studied system: 4-NTP
4.2.1 Electrochemical study of the reduction mechanism of a 4-NTP monolayer
4.2.1.1 Description of the studied system
4.2.1.2 Gold sphere electrode functionalization and experimental details
4.2.1.3 Gold sphere electrode characterization
4.2.1.4 Irreversible reduction of 4-NTP
4.2.1.5 Electrochemical characterization of 4-ATP
4.2.1.6 Study of the conversion of 4-NTP into 4-HATP
4.2.2 Spectroscopic characteristics of the system
4.3 Spectroelectrochemical analysis of 4-NTP reduction reaction
4.3.1 Electrochemical reduction of 4-NTP monitored by ex situ TERS
4.3.2 Electrochemical reduction of 4-NTP monitored by in situ EC tip SERS
4.3.2.1 Description of the setup and experimental conditions
4.3.2.2 Ex situ tip SERS
4.3.3 Potential dependent tip SERS measurements
4.4 Conclusion
5 Electrochemical TERS imaging of functionalized gold surfaces
5.1 Introduction
5.2 Description of the setup
5.2.1 Electronic implementation
5.2.2 Cell design for EC STM-TERS measurements
5.2.3 Tip insulation and characterization
5.3 Evaluation of the system stability
5.4 EC TERS imaging
5.4.1 Laser focusing on the tip
5.4.2 Preliminary results on EC-TERS imaging
5.5 Conclusion
6 TERS characterization of surfaces derivatized with diazonium salts
6.1 Introduction
6.2 Chemistry and electrochemistry of diazonium salts
6.3 Electrochemical grafting of a monolayer of a diazonium salt
6.3.1 Electrochemical grafting and characterization of a monolayer of penuorobenzene
6.3.2 STM imaging of PFBD-grafted gold surfaces
6.3.3 TERS analysis of the PFBD-grafted surface
6.4 Electrochemical and TERS investigation of an electroactive diazonium salt
6.4.1 Electrochemical characterization of a gold electrode functionalized by FeBTPD
6.4.2 STM-TERS analysis of a FeBTP grafted surface
6.5 Spontaneous grafting of diazonium salts investigated by STM-TERS .
6.5.1 Sample preparation and characterization
6.5.2 STM-TERS analysis of the spontaneously grafted layer
6.6 Conclusion
Conclusion and outlook
A Supplementary materials to Chapter 4
A.1 Raman signature of N,N-dimethylnitrosoaniline
B Supplementary materials to Chapter 6
B.1 Electrochemical characterization of a FeBTP functionalized gold sphere electrode
B.2 Raman Spectra of reference products
B.2.1 Raman signature of 2,3,4,5,6-pentauorobenzenediazonium tetra- uoroborate (PFBD)
B.2.2 Raman signature of 4′-(phenyl)-2,2′:6′,2-terpyridine (BTP)
B.2.3 Raman signature of [Fe (4′-(phenyl)-2,2′:6′,2-terpyridine) (4- ([2,2′:6′,2-terpyridin]-4′-yl)benzenediazonium)(PF 6 )2(BF 4 )2] (FeBTPD).
B.3 Synthesis and characterization of organic compounds
B.3.1 Synthesis of 2,3,4,5,6-pentauorobenzenediazonium tetrauoroborate (PFBD)
B.3.2 Synthesis of 4′-(phenyl)-2,2′:6′,2-terpyridine (BTP)
B.3.3 Synthesis of 4-aminobenzaldehyde
B.3.4 Synthesis of 4′-(4-aminophenyl)-2,2′:6′,2-terpyridine
B.3.5 Synthesis of 4′-(4-bromophenyl)-2,2′:6′,2-terpyridine
B.3.6 Synthesis of 4-([2,2′:6′,2-terpyridin]-4′-yl)benzenethiol
B.3.7 Synthesis of [Fe (4′-(phenyl)-2,2′:6′,2-terpyridine)2(PF 6 )2] (FeBTP) derivatives
B.3.7.1 Synthesis of [Fe (4′-(phenyl)-2,2′:6′,2-terpyridine)2- (PF 6 )2] (FeBTP)
B.3.7.2 Synthesis of [Fe (4′-(phenyl)-2,2′:6′,2-terpyridine)- (4′-(4-aminophenyl)-2,2′:6′,2 terpyridine)(PF 6 )2]
B.3.8 Synthesis of [Fe (4′-(phenyl)-2,2′:6′,2-terpyridine) (4-([2,2′:6′,2- terpyridin]-4′-yl)benzenediazonium)(PF 6 )2(BF 4 )2] (FeBTPD)
Abbreviations and symbols
Bibliography
