Raman Imaging enables the identification ofmolecules, their allotropes and polymorphs,the determination of their orientation, purityand crystallinity, and the detection of strainstates.

SEM/EDX allows for the identification ofatoms and chemical compounds and theimaging of surface structures.

RISE combines the advantages of both, thusfacilitating the most in-depthcharacterization of the sample.

RISE analysis of polymorphs:correlating structure withchemical phases

With SEM it is possible to identify materialsconsisting of different atoms using EDX(energy-dispersive X-ray spectroscopy). Itcannot however distinguish betweendifferent modifications of chemically identicalmaterials (polymorphs). As the manner ofatomic bonding greatly influences thestructure and properties of a material,visualizing not only morphology but alsoidentifying its molecular architecture isimportant. RISE microscopy accomplishes

both of these tasks as demonstrated with theanalysis of TiO2 polymorphs (fig. 1). TiO2 isstudied intensively because of its interestingchemical and optical properties and is widelyemployed in photocatalysis, electrochemistry,photovoltaics and chemical catalysis. It is alsoused as white pigment in tooth paste, sunscreen and wall paint and as anode materialfor lithium-ion batteries. Depending on theapplication, one crystalline form or the othergains in importance. TiO2 occurs in eightmodifications, two of which – anatase andrutile – were examined. For RISE microscopyan SEM image taken of a 1:1 anatase/rutilepowder mixture was merged with thecorresponding confocal Raman image (fig. 1a, c). It was demonstrated that though theelemental compositions of the modificationsanatase and rutile are identical, they can bedistinguished from one another by theircharacteristic Raman spectra at wavenumbers between 300 and 800 rel 1/cm (fig.1b). Both phases were combined in agglo-merates, in which rutile accumulated in largerparticles than anatase. To our knowledge thisis the first direct visualization of the spatialdistribution of anatase and rutile phases ofTiO2 in a mixture of both.

On the RISE: New Correlative Confocal Raman and Scanning Electron Microscopy

application note

Fig. 1: RISE microscopy of TiO2polymorphsTwo modifications of TiO2, anatase and rutile, were mixed 1.1, ground, dissolved in water and imaged with an SEM (a) and a confocal Raman microscope. Both images were thenoverlaid (c). In the Raman spectrum (b) anatase (blue) can be easily distinguished from rutile (red).Image parameters: 12 x 12 µm2 scan range, 150 x 150 pixels = 22,500 spectra, integration time: 0.037 s/spectrum.

Knowledge of the morphology and chemicalcomposition of heterogeneous materials onthe sub-micrometer scale is crucial for thedevelopment of materials with newproperties for highly specialized applications.Each analytical microscopy technology hasits own distinct advantages and limitations.For comprehensive characterization ofmaterials, a new correlative technology hasbeen developed: RISE microscopy. ConfocalRaman Imaging for chemical analysis andScanning Electron Microscopy for ultra-structural analysis are now combined withina single instrument. Measurement positionsare automatically retrieved, rendering thetransfer of samples between microscopesobsolete and leading to perfectly overlappingRaman/SEM images.

Why RISE microscopy?

RISE Microscopy is the combination ofconfocal Raman Imaging and ScanningElectron Microscopy. It incorporates thesensitivity of the non-destructive, spectros-copic Raman technique along with the atomicresolution of electron microscopy. EDX (energydispersive X-ray spectroscopy) can also beintegrated.

(a)

Raman Raman-SEM overlay

b) c)

b)a) c)

Fig 3: Three-dimensional RISE imaging of SWCNTs on a PE filterThe confocal setup of the Raman microscope allows the acquisition of 3D Raman images by moving the focus of theobjective through the sample. For this picture (b) 16 equidistant Raman images were taken while the focus wasshifted by 1 µm for each image.The individual images are displayed as RISE images in (a). The compiled 3D Ramanimage of the analyzed sample area shows the SWCNTs located on the top of the PE filter (b).Image parameters: 200 x 200 µm2 scan range, 70 x 70 pixels = 4,900 spectra, integration time: 0.037 s/spectrum.PE (red), SWCNTs (green), SEM images (grey).

Fig. 2: RISE imaging of SWCNTs on a PE filterCharacteristic Raman spectra of SWCNTs and PE (a), color-coded Raman image (b) and RISE image (c).Image Parameters: 150 x 150 µm2 scan range, 80 x 80 pixels = 6.400 spectra, integration time: 60 ms/spectrum.PE (red), SWCNTs (green), SEM image (grey).

RISE analysis of allotropes:Single-wall carbon nanotubes

Carbon nanotubes are cylindrical allotropemodifications of carbon. Their specialproperties such as high thermal conductivitymake them interesting for applications inelectronics, nanotechnology, optics andmaterials science.

Single-wall carbon nanotubes (SWCNTs)consist of a single atom thick, curved sheetof carbon. Their unique electronic andmechanical properties make them attractivefor electronics fabrication. The Raman spectrum identifies SWCNTs as

single-walled by the presence of theircharacteristic RBM (radial breathing mode)bands at low wave numbers (fig. 2a). The RISEimage of SWCNTs embedded in polythylene(PE) (fig. 2c) was generated by merging theRaman image (fig 2b) with the SEM image.

a)

a) b)

To explore the 3D structure of the material ,16 Raman images were taken in the z-direction intervals of 1 µm (fig. 3a) From thesedata a 3D image was compiled. It illustratesthat the SWCNTs lie on top of the PE matrix(fig. 3b).

b) c)a)

SWCNTRBM

PE

The RISE microscope above consists of a WITec confocalRaman microscope combined with a Tescan SEM.

application note

Fig. 4: RISE imaging of MoS2 crystalsA color-coded Raman image was overlaid on an SEMimage (a) to give the RISE image of MoS2 twin crystals(b). Spectra of MoS2 monolayers (red), two or morelayers (green) and silicon (purple) are presented in (c).Image parameters: 22 x 17 µm2 scan range, 65 x 50pixels = 3,250 specra, integration time: 0.037s/spectrum.

Fig. 5: In situ characterization of electron beam inducedmodifications of WS2A photoluminescence image (a) and its PL spectra (c)before modification; the same crystal after modificationat the areas indicated by crosses (b) with correspondingspectra in PL (d). Spectra (d) are color coded accroding tothe energy of the electron beam: 0 kV (yellow), 0 kVborder (cyan), 1 kV (red), 2 kV (blue), 5 kV (green), 10 kV(black).

RISE microscopy of 2D materials

Thin-layered or single layer materials, definedas 2D materials, have recently attractedenormous research interest due to theirspecial electronic and optical properties whichdiffer significantly from that of their bulkprecursors. Inspired by progress in grapheneresearch, other mono-layered materials suchas hexagonal boron nitride (h-BN) andtransition metal dichalcogenides (TMDs) havealso received widespread attention. Recentwork has shown that exfoliated monolayermolybdenium disulfide (MoS2) is a 2D directbandgap semiconductor indicating that thematerial is suitable for optoelectronics andenergy harvesting. Bulk MoS2 however is anindirect bandgap semiconductor. Thusdetailed knowledge of the structures andfeatures of grains and grain boundaries areessential for understanding and exploringthe materials’ properties and its furtherapplications. Here we present with MoS2 andtungsten disulfide (WS2) that RISE microscopyreveals structure as well as crystalline andexciton dynamics of thin-layered TMDs.

MoS2 twin crystalsCVD grown monolayers of TMDs formtriangular two dimensional crystals. Twincrystals of MoS2 on SiO2/Si appear in the SEMimage as star-shaped forms (fig 4a). TheRaman spectra of these 2D crystals show thecharacteristic E2g and A1g Raman band modesof MoS2 (fig. 4b). With an increasing numberof layers the two Raman bands drift apartdue to inter-layer and in-plane vibrations.

At the grain boundaries the Raman bands notonly show shifts, but additional bands alsoappear, indicating defects or misaligned 2Dcrystals. They probably result from adjacentcrystals colliding out at their boundaries. Theoverlapping boundaries identified by RamanImaging correlate perfectly with the darkedges visible in the SEM image.

In situ modification of WS2

In addition to characteristic Raman spectraTMDs show strong photoluminescence (PL)in the visible range making them promisingmaterials for optoelectronics. PL emission isattributed to excitation recombination. ThePL - therefore the optical properties - of WS2can be modified using the electron beam ofthe SEM.

A monolayer WS2 triangular island showsphotoluminescence at roughly 640 nmwavelength (fig. 5a, b). Defined 2x2 µm2 areasof this WS2 crystal were then scanned atincreasing electron acceleration voltages in

the SEM. PL was then measured again (fig.5b, d). Surprisingly PL changed as a functionof the previously applied electron accelerationvoltage. The most distinct change in PL wasseen at 1,2 kV, most likely because at higherkV electrons were being absorbed while atlower kV they were totally reflected. As PL isa measure of optoelectronic properties ofTMDs, RISE microscopy enables theirmodification and analysis in situ. Afterreleasing the sample from vacuum this effectmay disappear.

a)

Raman

SEM

a)

SEM

Raman

a) b) c)

d)

a) b)

c)