The growing amount of applications for nano-fibers, particularly those created through the process of electrospinning, produces a demand for imaging such fibers in order to view their subtle morphology along with their diameter and alignment.
Fibers produced via electrospinning are utilized in a number of disciplines from environmental engineering (filters and membranes), healthcare (drug delivery and tissue engineering), to energy storage (fuel cells and solar cells). They are often processed more by using chemical modification to introduce active molecules to their surface.
Prior to this occurring, it is crucial to evaluate their original surface as the mechanical features of a material is heavily influenced by the structure. Understanding the subtle structure and morphology of the fibers assists in managing the post-treatment process in the most effective manner.
As the diameter of the fiber is nanoscale in size, the application of ultrathin, high quality conductive coating is required for high magnification electron microscopy imaging.
Electrospinning fibers comprise of polymers, producing stacks in which the fibers are in contact with each other but are disconnected. This creates challenges during imaging with an electron beam.
An ultra-thin and dense metal coating will not only enhance the signal and eliminate charge from the sample, it will additionally stabilize the fibers and create some rigidity (Figure 1). Charge is readily gathered by uncoated electrospinning fibers, even during imaging with a low voltage (Figure 2).
Figure 1. PvDF electrospinning fibers coated with 1 nm of gold with use of Quorum Q150V Plus coater. Image Credit: Quorum Technologies
Figure 2. SEM image of uncoated PvDF electrospinning fibers, poor contrast and charging effect is visible. Image Credit: Quorum Technologies
This causes them to move beneath the beam. A sufficient coating will eliminate the charge, enhance contrast, stabilize the fibers and mitigate damage to the sample. It will additionally help in protecting the microscope from contamination that may arise from the sample, such as when the fibers are modified with particles.
The Quorum Q150V Plus coater was employed to produce all the ultra-high quality coatings. This is designed for high-vacuum applications with a maximum vacuum of 1x10-6 mbar. A Hitachi SU8230 scanning electron microscope was utilized to image the coated samples.
Desired Coating Characteristics
For SEM imaging, a conductive coating must contain two main characteristics: firstly, the coating layer must be adequately thin so it does not cover the morphology of the sample, secondly, it must be dense enough to offer conductivity, which enables any accumulating charge to be eliminated.
Figure 3. Tear on PvDF electrospinning fiber. Image Credit: Quorum Technologies
Figure 4. The structure of damaged fiber, fibers coated with 1 nm of gold with use of Quorum Q150. Image Credit: Quorum Technologies
When selecting from the range of coating materials available, several factors should be considered.
Firstly, when sputtered under the same vacuum, different metals show a different grain size, which is demonstrated in Figure 5.
Figure 5. 2 nm of gold, platinum and iridium sputtered onto silicone surface, base pressure 1E-06 mbar, sputtering current 20 mA. Image Credit: Quorum Technologies
In the coating, the selected metal must not contain bigger grains than the coated sample’s features, or else the coating will shield the morphology of the sample.
Secondly, the metal must not be too brittle and must offer a very good signal enhancement. The fibers will move even if a light stress is applied to them. This can result in the cracking of brittle metal, creating a number of small features which can be inaccurately observed as fiber morphology. This is shown in Figure 6.
Figure 6. PvDF electrospinning fiber coated with 2 nm of iridium, base vacuum 1E-06 mbar, cracks caused by stress. Image Credit: Quorum Technologies
The grain size of sputtered metal can be influenced by modifying the process parameters when utilizing a cold magnetron sputtering technique for coating samples.
The sputtering current, sputtering gas pressure, and base vacuum perform a crucial role in the coating process and heavily impact the quality of the coating. This is the case for any target material employed.
The vacuum is the component that performs the most crucial role in coating. The grains of the coated material are finer when an effective vacuum is used. This is because less particles are available that the sputtered metal can run into on its route to the surface of the substrate. A smaller amount of water vapor, oxygen, and nitrogen also mitigates chemical reactions throughout the sputtering process. The coatings produced are dense, have a very fine grain size and hold no defects or impurities. This enables coverage with minimal thickness of metal and provides high contrast SEM images, which explicitly demonstrates their surface morphology.
To demonstrate the effect of sputtering parameters on the quality of the coating, the gold coating was selected as it contains all the desired features: a clearly distinct grain size created under various vacuums, good mechanical stability, and a good SE yield.
Base Vacuum Level
The base vacuum is a parameter that has an instant effect on the quality of the coating. Different grain sizes will be produced when the same metal is sputtered under various base vacuums.
This effect is portrayed in Figures 7A, B and C. Each of the coatings were produced with an equal thickness: 2 nm with an equivalent sputtering current (20 mA) and an equivalent pressure of sputtering gas (Ar) 10-2 mbar but with an alternative base vacuum.
Figure 7. Influence of base vacuum on the coating quality. Image Credit: Quorum Technologies
Cold magnetrons enable the utilization of low currents in sputtering. This enhances coatings regarding the grain size and the coverage density. Low sputtering currents provide a smaller grain size and when combined with a high base vacuum, they provide highly dense layers of small thickness.
Low sputtering current is particularly beneficial where there is a requirement to access cavities and hard-to-reach areas for example in electrospinning fiber scaffoldings.
Figure 8 demonstrates the effect of the base vacuum and sputtering current on the gold coatings produced. Each of the coatings had a thickness of 2 nm.
Figure 8. Influence of base vacuum and sputtering current on coating quality. Image Credit: Quorum Technologies
As mentioned previously, electrospinning fibers form stacks but are not connected. These samples are the most difficult for SEM imaging because fibers gather charge and can move when exposed to the electron beam.
The applied coating must be sufficiently thin to avoid obscuring its subtle morphology when imaging the surface of these fibers.
The general rule is ‘less is more’ in EM imaging. More surface details can be observed with thinner coatings, as shown in Figure 9. The use of low sputtering current, sufficient sputtering gas pressure, and high vacuum will enable coverage of fibers with coating thickness as small as 1 nm.
Image Credit: Quorum Technologies
The structure of a single fiber intrinsic to the process of electrospinning is revealed by a coating thickness of less than 2 nm. This enables the electrospinning parameters to be modified in order to attain the smoothest fibers which are free from artifacts.
When a clear image of the modification material is available, the additional modification of fiber samples can be managed better. Figure 9 provides an example of appropriate coating thickness and ‘too much’ material to view the fine modification of the fiber.
Figure 9. PAN electrospinning fibers modified with nanoparticles, coated with 5 nm of gold with use of base vacuum 2x10-6 mbar , too thick coating obscures tiny crystals that the fibers were modified with b) PAN electrospinning fibers modified with nanoparticles, coated with 2.5 nm of gold with use of base vacuum of 2x10-6 mbar. Image Credit: Quorum Technologies
There is often a requirement to image the structure of the modification-precipitating crystals of the fibers or their agglomeration along with the fiber structure.
Dense and thin coatings enables researchers to take clear and crisp images of both crystals and fibers, which is portrayed in Figure 10. Each of the electrospinning fiber samples were supplied by the University of Warwick.
Figure 10. PAN electrospinning fibers modified with nanocrystals, sample coated with 1 nm Au/Pd with sputtering current 1 mA, base vacuum 7x10-7 mbar. Image Credit: Quorum Technologies
This information has been sourced, reviewed and adapted from materials provided by Quorum Technologies Ltd.
For more information on this source, please visit Quorum Technologies Ltd or contact Anna Walkiewicz on [email protected].