Displaying items by tag: AFMRaman

Silicon

SECM with Raman Scattering in situ

Nanonics systems provide unique liquid-based capabilities in scanning electrochemical microscopy (SECM) that now for the first time can be combined with other SPM methods such as AFM-Raman for chemical imaging together with the topographic and electrochemical current imaging.  The Nanonics SECM capability incorporates Nanonics innovations in probe design, tip-sample feedback, and liquid cell design to enable new and revolutionary capabilities for the most advanced experiments.

Nanonics manufactures custom SECM probes with a continuous nanowire of platinum embedded in glass.  A side view is shown below on the left left while a top view is shown below on the right clearly showing the platinum wire and glass.  In the top-view, the white spot in the middle is the wire while the black ring around it is the glass.

Click the images below to download a detailed PDF description:

These custom probes provide simultaneous normal force sensing with full SECM functionality.  

Furthermore Nanonics provides a custom-designed liquid cell and environmental chamber to use in such measurements. 

For SECM measurements, the Raman setup through the laser, spectrometer, and CCD camera are placed above the probe with the optically friendly scanner and probe. Fluid measurements are doing using the critically important water immersion objective.  The placement of the SPM controller for AFM measurement and Potentistat for the SECM current measurements are shown on the right of the schematic.

SECM-Raman Application:  Simultaneous SECM current and Raman imaging of copper during real-time etching

A silicon wafer with a thin layer of copper was used as the substrate for this electrochemical etching experiment.   The SPM probe etched a small, ~4 um hole within the copper layer exposing the silicon substrate.   Images of the substrate before (left)  and after (right)  the etching can be seen below with the etched point showing up as a dark spot in the right image.

The etching was monitored in real time with in situ Raman scattering where the Raman signature of silicon at 523 cm^-1 was used to track the appearance of the silicon and thus progress of the etching process.  A sample spectrum revealing the Si peak in the Raman spectrum is shown here: 

This paper by Spanish researchers Navarro-Suarez et al.  describes the development of a new technology to produce lignin-based nanoporous carbon with narrow and tunable pores. This novel material was prepared by chemical activation of natural lignin at 900˚C, and it was tested as an electrode material for electrical double-layer capacitors.

Characterization of the morphological, chemical and electrical properties of this new material was done by different methods including SEM, HRTEM, EDX, electrochemical analysis and Raman microscopy.  Raman characterization was performed with a Nanonics Multiview 2000 integrated with a Raman spectrometer. Such an integrated system enables characterization of morphological properties with simultaneous chemical characterization.  

The authors shows that there are ordered and disordered carbon regions and presence of few layer graphene (FLG) embedded into the amorphous carbon matrix.  The Raman spectral imaging and ratio between I_D and I_GRaman peaks of carbon confirmed SEM and HRTEM morphological results.  Further electrochemical studies showed that capacitance value of 6.87 Fg^-1 can be obtained.

Thus, the obtained results demonstrated that nonporous carbon with tunable pore size can be achieved by using a natural material such as lignin and can be effectively used as electrodes material in energy storage applications. 

Distribution obtained by Raman spectral imaging of the ordered and disordered areas inside a particle for samples (a) 2, (b) 4 and (c) 6.

Color scale on the right varies with the ID/IG values from 0.0 (dark blue) to 1.2 (dark red).

Published:  RSC Advances, 2014, 4, 48336-48343

Click here for more information on the MultiView 2000 system

Graphene

Differentiating single, double, and multi-layers of Graphene

The Nanonics MultiView series (MV 1000, MV 2000, and MV 4000) provide complete hardware and software integration for the full suite of AFM and Raman characterization of all your materials including AFM, Raman, TERS, and NSOM characterization.   AFM together with Raman based methods have become the method of choice for characterizing carbonaceous materials.  For graphene, AFM-Raman imaging is especially useful for differentiating single, double, and multi-layers of graphene.

AFM-Raman of Graphene

A series of images of graphene flakes is shown. The first image (a) is an optical top-view of the graphene flake on silicon, collected with an upright optical microscope.  Next is a 10um x 10um AFM height image (b) of the graphene flake showing 2 distinct areas.  Figure (c) shows Raman spectra collected from the different areas, clearly differentiating the region of the single layer (red spectrum) and the double layer (blue spectrum.)  Finally, a cross-sectional profile across one of these layers is shown in (d) showing a 6 angstrom step height, corresponding to the thickness of a single graphene flake.

In addition to Raman spectra being collected at individual points, AFM-Raman imaging enables the collection of a Raman image at a particular wavelength where the intensity of the Raman peak at that wavelength is mapped over the surface, simultaneously collected with topography from conventional AFM.  In this way, an intensity map at the different Raman markers can be collected. For example, the 2676cm^-1 band corresponding to the 2D band for the single layer or the 2700 cm^-1 band corresponding to the double layer band can thus be mapped.  

This has been done in the images below, where the Raman intensity has been overlaid on top of the 3D topography where red shows the highest Raman intensity and green is the lowest Raman intensity [blue coloring corresponds to region without any graphene].   On the left in (a) is the Raman intensity map of the 2676cm^-1 single layer band, showing highest distribution of the single layer in the triangular red zone in the top half of the image. On the right is the AFM-Raman intensity map of the same area as (a) but now of the 2700cm^-1 double layer band, showing the strongest signal in the top trapezoidal and bottom rectangular orange regions.  Note that the single and double layer band regions are located in different areas of the surface and are easily identified using the Raman maps.

Tip-enhanced Raman spectroscopy (TERS) of graphene

Nanonics unique AFM-Raman integration allows for new horizons in graphene characterization with Tip Enhanced Raman Spectroscopy (TERS).  In TERS, a metallized sharp tip is used to enhance the electric field signal at the tip’s end.  Unique TERS probes are provided by Nanonics that contain a gold nanoparticle embedded in the surrounding glass; this kind of probe gives high dielectric contrast with a well-defined plasmonics resonance and without a Raman background.  When combined with tuning fork (laser-free) feedback, this probe provides a completely free optical axis for top-viewing of opaque samples. Additionally, these probes provide considerable enhancement and so the use of these probes does not require a conductive substrate for Raman enhancement.  Finally, the Multiview 2000 and Multiview 4000 offer both tip and sample scanning scanners, enabling quick and easy hotspot detection by tip positioning.  The combined tip and sample scanning stage provides accurate positioning of the probe at the ideal location with respect to the focused laser spot and the sample can be scanned without losing the Raman hotspot position.

The data bellows shows an optical image on left of a TERS probe in top-view positioned on Graphene single and multi layers.   The corresponding far-field (Black) and near-field enhanced (Red) spectra are shown on the right where the probe was in contact with the graphene single layer, resulting in the red TERS spectrum.  The TERS spectrum clearly shows that only some of the Raman peaks are enhanced over the far-field spectrum, specifically the 1589cm^-1 and 2677cm^-1 peaks are enhanced.  This shows that specific vibrational modes in the graphene are locally enhanced near the tip. Thus, the vertical resolution of the Raman data is significantly improved with TERS, which now enables distinction between the substrate and the upper layers.

Moving the tip to a different position on the surface as shown in the optical image below on the left, the interaction of the probe with the sample now reveals multilayer graphene, as revealed in the black (far-field) and red (TERS) spectrum on the right.

Raman Difference Spectroscopy

Nanonics has implemented into the Multiview 2000 and 4000 series a set of specialized AFM scanning protocols of Raman difference spectroscopy effectively discriminating between the near-field and far-field contribution of the scattered Raman signals. Such Raman difference protocols provide a selective enhancement of graphene single layer in-plane vibrational modes with lower effect of subsequent graphene layers.  Moreover, such scanning protocols allow for maximizing Z polarization excitation even when using conventional Raman excitation with a Gaussian focused laser beam. 

Below is an AFM image of graphene in i), where both far-field Raman (black), TERS (red) , and difference (green) spectra have been collected at a variety of spots on the surface.  Spectra in ii) were collected at points (a) revealing a single layer of graphene.  The spectra in iii), collected at point (b) reveal a double layer of graphene while the spectra in iv) collected at point (c) reveals 3-4 layers.  This kind of difference mapping helps to identify the enhanced band only.

Other  Multifunctional Graphene Imaging

Other important properties of graphene can be probed as well. Below, we show thermal and Kelvin probe images of graphene, collected simultaneously with height so that correlation with the topography is possible.  AT bottom is a schematic showing the setup to probe the near-field photoconductivity of graphene.

1. Thermal-conductivity Imaging of Graphene

AFM (left) and Overlaid AFM/Thermal imaging (right) of graphene flakes performed with Nanonics Thermo-conductivity probe

2. AFM and Kelvin Probe Imaging of Graphene

AFM (left) and KPM (right) images of graphene transistor with opposite voltage bias

3. Near-field Photoconductivity of Graphene

Carbon Nanotubes

Mapping and differentiating CNTs

The Nanonics MultiView series (MV 1000, MV 2000, and MV 4000) provide complete hardware and software integration for the full suite of AFM and Raman characterization of all your materials including AFM, Raman, TERS, and NSOM characterization.   AFM together with Raman based methods have become the method of choice for characterizing carbonaceous materials.  For carbon nanotubes, AFM-Raman imaging is especially useful for differentiating the different kinds of nanotubes.

Below is a 10um x 10um AFM topography image of carbonaceous material on silicon clearly showing the presence of a nanotube on the surface in addition to some round carbonaceous domains at the top and bottom two corners.  

Raman spectra were collected both on the nanotube and the circular domains.   Below the red spectrum corresponds to the Raman spectrum on the nanotube showing a large peak at 1575cm^-1 and a smaller peak at about 1340cm^-1.  A Raman spectrum from the round domains is shown in green revealing a large peak at 1340cm-1 and a weak, dual peak at about 1600cm^-1.  These spectra clearly differentiate the carbon nanotube and the surrounding disordered material. 

Finally, a Raman intensity map at 1600cm^-1 and 1340cm^-1 was collected with the resulting image shown below, clearly mapping the carbon nanotube (red) and the regions of disordered materials (Green).

Raman difference spectroscopy

Nanonics has implemented into the Multiview 2000 and 4000 series a set of specialized AFM scanning protocols of Raman difference spectroscopy effectively discriminating between the near-field and far-field contribution of the scattered Raman signals.   The upper scanner in the scanning head enables automated vertical tip motion at every pixel during the AFM/Raman mapping.  Thus, a Raman map can be collected while the tip is in contact with the sample (near-field signal) and then again when the tip is retracted at a predetermined distance above the sample (far-field signal) and then retraces the substrate.  The software then automatically calculates the difference between the intensity of the Raman spectrum when the TERS probe is in contact and when the probe is retracted.

A Multiview 4000 with a TERS probe was used to collect Raman difference spectra of carbon nanotube clusters in the images shown below. A topography map of the clusters is shown in (a) revealing several hundred nanometer round domains of carbon nanotubes. 

                      (a)

Next a TERS map with the probe in contact with the sample was collected in (b) showing the Raman intensity at 405cm^-1.   The Raman map was collected at an array of 1850 points (pixels shown in the topography image) where measurements were separated by 180nm.

The tip is then retracted, and the far-field Raman intensity map at 405cm^-1 is again collected as shown in (c).   The difference map between (b) and (c) is shown in (d), clearly providing significantly better resolution in the Raman intensity map that can be accurately correlated with the topographic map in (a), revealing important chemical information about the various CNT clusters.