AFM-Raman is now a well established technique offering a multi-technique platform for deeper understanding of materials at the nanoscale. Tip-enhanced Raman scattering (TERS) takes advantage of that same platform combined with surface plasmon resonance effects localized at the probe-tip to bring chemical information with nanometric spatial resolution. These techniques have the potential to transform spectroscopic research and sample characterization in many varied fields, including nanomaterials and material sciences, semiconductors, life sciences.

The examples given below illustrate the power of NanoRamanTM and TERS analysis to provide new insights into sample structure. These examples should not be considered exhaustive; a list of TERS publications using HORIBA Raman spectrometers can be found here

TERS of 1D materials: Nanotubes and Nanowires

Why TERS? to give a fundamental insight into the optical and electronic properties of 1D materials.

The synthesis of carbon nanotubes (CNTs) results in bundles that often contain a variety of tubes with different structural properties. Conventional Raman is widely used to characterize quasi-one dimensional structures such as CNTs, however the optical imaging of their nanoscale properties is severely restricted to the optical diffraction limit. A good illustration of how TERS can be used to gain information that would not have been accessible with conventional Raman can be seen in the following TERS map.

TERS Chemical mapping of a single nanotube.
Figure. TERS Chemical mapping of a single nanotube. 300 x 160 nm (100 x 60 points); Excitation 638 nm; 0.13 mW; integration 0.1 s. Left. TERS map of a single CNT. Right. TER spectra of the CNT. (Data obtained in our Application Lab.)
 

 

In this TERS map, the intensity of the D band (blue pixels) is showing the imperfection in the structure of the latter; in contrast the areas in red correspond to the pure graphitic arrangement of the CNT through the intensity of the 2D band.

Defects concentration, local chirality changes from the different radial breathing modes, pressure effect and strain distribution has been studied at the single carbon nanotube level through TERS. Si, Ge and GaN nanowires are also candidates for TERS.

TERS imaging of 2D materials: Graphene, Graphene Oxide, MoS2

Why TERS? to characterize graphene in terms of size, shape, electronic properties, distribution of defects and contaminants.

The discovery of graphene in 2004 has given rise to a surge in research activities on 2-D materials with novel properties and the need to characterize them at the nanoscale. As for the 1D materials, TERS gives localized information down to 10 nm through the Raman signature and allows the identification of the defects (point defects, vacancies or dopants). More than the identification, thanks to the TERS imaging, the localization changes of the defects concentration in graphene and graphene oxide is possible.

TERS Chemical mapping of Graphene Oxide flakes.
Figure. TERS Chemical mapping of Graphene Oxide flakes. a) 1 x 1 µm2 TERS mapping of GO-COOH on Au; Excitation 638 nm; integration 0.1 s. b) Typical TER spectra from flat area of the GO flake, crease and fold. (Data obtained in our Application Lab.)
 

 

Monitoring the defects of the graphene flakes and their concentration (via the distribution of the ratio of G to D band intensities) is of great interest to study the impact on the design of devices made with such materials.

Graphene, but also any functionalized and decorated 2D materials, Molybdenum Disulfide, Tungsten Disulfide are good candidates for a TERS study!

TERS for probing just a few molecules!

Why TERS? the chemical characterization of molecules on a surface is crucial for understanding the reactivity and function of molecules.

In surface chemistry, catalysis or biology, the chemical characterization of molecules on a surface is crucial for understanding their reactivity and function. However, the small quantity of molecules present renders their spectroscopic characterization challenging and most of the time impossible by confoncal Raman.

TERS map obtained monitoring the 1141 cm −1  peak of  a patterned azobenzene SAM formed by nanocontact printing.
Figure. (a) TERS map obtained monitoring the 1141 cm −1 peak of a patterned azobenzene SAM formed by nanocontact printing. (b) TER spectrum of the azobenzene molecules. Excitation 633 m; laser power 0.5 mW. (Courtesy of Dr Marc Chaigneau.)
 

 

As can be seen in this figure, TERS, especially its gap-mode realization, proved to be an efficient tool for detection and nanoscale Raman mapping of molecular layers down to single molecule layers self-assembled on the surface (SAM). Imaging the chemical species at the nanoscale gives an insight on the SAM grafting quality (in this example after a nano-stamping process) as well as informations on the phase segregation.

TERS is thus a great candidate to image organic molecules or polymer blends. It allows distinguishing the nanoscopic domains with distinct chemical signature, but also studying the photocatalysis and others Plasmon-induced catalysis processes.

TERS Imaging of semiconducting nanostructures

Why TERS? Monitoring the local stress/strain in semiconducting nanostructures and the impact on the electronic properties.

Characterization of the strain present in semiconductor nanostructures such as strained Si (sSi) on insulator (sSOI) or epitaxially grown SixGe1-x on Si (SiGe/Si) is of special interest for the microelectronics industry.

The stress state in crystal materials can be determined by using techniques such as X-ray diffraction, transmission electron microscopy and Raman spectroscopy. The main advantages of the Raman technique over the first two are its non-destructive nature and no need for sample preparation. The presence of strain breaks the crystal symmetry and shifts the frequencies of the Raman modes with respect to the unstrained ones thus making possible the determination of the stress state. Anyway Raman spectroscopy only gives an average value of the strain, and the optical resolution is not suited to current problems of the microelectronics industry.

Integrated intensity of the Si-Ge TERS peak as a function of the tip across a SiGe nano-stripe
Figure. (a) Integrated intensity of the Si-Ge TERS peak as a function of the tip across a SiGe nano-stripe. (b) Stress-induced Si-Ge phonon shift as a function of the TERS tip position across the nano-stripe. (Courtesy of Dr Marc Chaigneau.)
 

 

TERS allows studying the misfit strain down to the nanoscale in the present example of 150 nm lithographically defined SiGe nano-stripes on Si(001) substrate. During the TERS experiment, the intensity and the frequency of the locally enhanced Si-Ge and Ge-Ge Raman modes across a single nano-stripe are monitored, giving the perpendicular strain profile with a lateral resolution of ~ 20 nm. The strain is tensile and becomes maximum (~+1.4 %) at the centre of the nano-stripe, decreasing close to zero at the edges.

TERS studies on Si, SiGe but also on CdS, GaN and GaAs have been also published by using HORIBA Raman spectrometers.

 
 
 
 

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