Scanning transmission X-ray microscopy (STXM) is a type of

X-ray microscopy An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce magnified images of objects. Since X-rays penetrate most objects, there is no need to specially prepare them for X-ray microscopy observations. Unlike visible li ...

in which a zone plate focuses an X-ray beam onto a small spot, a sample is scanned in the focal plane of the

zone plate A zone plate is a device used to focus light or other things exhibiting wave character.G. W. Webb, I. V. Minin and O. V. Minin, “Variable Reference Phase in Diffractive Antennas”, ''IEEE Antennas and Propagation Magazine'', vol. 53, no. 2, ...

and the transmitted X-ray intensity is recorded as a function of the sample position. A stroboscopic scheme is used where the excitation is the pump and the synchrotron X-ray flashes are the probe. X-ray microscopes work by exposing a film or charged coupled device detector to detect X-rays that pass through the specimen. The image formed is of a thin section of specimen. Newer X-ray microscopes use

X-ray absorption spectroscopy X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation facilities, which provide intense and tunabl ...

to heterogeneous materials at high spatial resolution. The essence of the technique is a combination of spectromicroscopy, imaging with spectral sensitivity, and microspectroscopy, recording spectra from very small spots.

Benefits of STXM

Radiation damage

Electron energy loss spectroscopy In electron energy loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their pa ...

(EELS) in combination with

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

has modest spectral resolution and is rather damaging to the sample material. STXM with variable X-ray energy gives high spectral resolution. Radiation damage effects are typically two orders of magnitude lower than for EELS. Radiation concerns are also relevant with organic materials.

Samples with water

Unlike other methods such as electron microscopy, the spectra samples with water and carbon can be obtained. STXM run at atmospheric pressure allows for convenient sample installation and fewer restrictions on sample preparation. Cells have even been built which can examine hydrated precipitates and solutions.

Operation

In order to obtain spectromicroscopy data the following operating procedure is followed. The desired

monochromator A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', "si ...

grating is selected along with

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

energy in the middle of NEXAFS range. Refocus mirrors are adjusted to put the beam into the microscope and steered to maximize the

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...

passing through the zone plate. A pinhole is placed in the photon beam upstream in a transverse position to maximize transmission. Pinhole size is determined by demagnification to the size of the diffraction limit of the zone plate lens. An undersized pinhole is often used to reduce intensity which controls radiation damage. The order sorting aperture is positioned to eliminate transmission of unfocused zero order light, which would blur the image. Then an x/y line scan is defined across an intensity variation in the image. The x/y line scans are repeated with varying focus conditions.

Adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

spectra can also be obtained with a stationary photon spot.

Applications

Quantitative polymer analysis

STXM has been used to study reinforce filler particles used in molded compressed

polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...

foams in the automotive and fishing industries to achieve higher load bearing capability. Two types of polymers, copolymer styrene and acrylonitrile (SAN) and aromatic-carbamate rich poly-isocyanate poly-addition (PIPA), are chemically indistinguishable by transmission electron spectroscopy. With NEXAFS, spectra of SAN and PIPA absorb strongly at 285.0 eV associated with the phenyl groups of the

aromatic In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...

filler particles and thus show the same electron spectroscopy image. Only SAN has a strong absorption at 286.7 eV due to the acrylonitrile component. NEXAFS can be a quick and reliable means to differentiate chemical species at a sub-micron spatial scale.

Distribution of macromolecular subcomponents of biofilm cells and matrix

STXM which uses near-edge X-ray absorption spectroscopy is able to be applied to fully hydrated biological molecules due to the ability of X-rays to penetrate water. Soft X-rays also provide spatial resolution better than 50 nm which is suitable for bacterial and bacterial microfilms. With this, quantitative chemical mapping at a spatial scale below 50 nm may be achieved. Soft X-rays also interact with almost all elements and allow mapping of chemical species based on bonding structure. STXM allows for study of a variety of questions regarding the nature, distribution, and role of protein, carbohydrate, lipid, and nucleic acid in biofilms, especially in the

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide struc ...

. The study of these

biofilms A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...

is useful for environmental remediation applications.

References

{{Reflist Scientific techniques Accelerator physics