Studying Nanomaterials: Spatial Resolution
When studying nanomaterials, the scale at which you are looking dramatically affects your results. There are three main paradigms for studying nanomaterials: ensemble measurements, which average data across multiple nanoparticles/nanoflakes; microscopy measurements, which can isolate individual nanoflakes/nanostructures; and tip-enhanced measurements, which can isolate individual atoms. There is some fluidity between these groupings; for instance, microscopy techniques using electrons can detect individual atoms. However, this paradigm is generally applicable.
We will start with ensemble techniques, which involve shining light onto a sample with limited to no focusing. This technique allows researchers to observe the properties of a sample made up of many nanoparticles (sometimes referred to as a bulk sample). Each individual nanoparticle will be slightly different due to small changes in size and structure and when you measure all of the particles simultaneously, you will observe the average of their properties. This is useful when studying a macroscopic sample made up of many nanoparticles, like in a solar cell.
Optical Microscopy
However, what if we want to study the individual nanoparticles within our sample? In that case, we need to focus the light, usually using an objective. We call this microscopy. In microscopy, our resolution (the smallest structure we can isolate and study) is limited by what is known as the diffraction limit. The diffraction limit is a restraint imposed by physics that limits our resolution to about half the wavelength of light. Given that visible light is around 400-800 nm, our resolution is usually limited to 100’s of nanometers. This allows us to isolate individual nanomaterials and some nanostructures.
Now, what if we want to get around this physical limit and study individual atoms? For that, we can use two different techniques. The first is a form of microscopy where we image materials using electrons instead of light. In these experiments we shoot electrons at a sample and collect the ones that are either transmitted or diffracted. Since electrons have sub-nanometer wavelengths, the diffraction limit for the electrons results in sub-nanometer resolution, allowing us to resolve individual atoms.
Top-Enhanced Technique
We can also use what are known as tip-enhanced techniques. In these setups, a nano-scale tip is kept within a few nanometers of the sample surface. Light is focused onto the sample and tip and evanescent fields (fields that do not propagate away from the sample) are detected. These fields are able to break the diffraction limit discussed earlier, also allowing for atomic scale resolution.
When picking a technique to use, you may think that increased resolution is better; however, each technique has its trade offs. Ensemble measurements do not allow you to assess individual contributions from each particle/flake, but are generally faster and easier to implement. In contrast, tip-enhanced techniques give you detailed information on individual nanoparticles and structures, but are slow and often difficult to use. Scientists need to weight these pros and cons and decide which technique, or combination of techniques, helps them achieve their goals.
Here are some examples of each technique and their potential use cases:
Tip-enhanced Techniques:
Atomic-Force Microscopy (AFM): determine the thickness and structure of materials
Kelvin Probe Force Microscopy (KPFM): determine the work function across a material
Scanning Near-Field Optical Microscopy (SNOM): detect evanescent waves
Microscopy Techniques:
Pump-Probe Microcopy: measure electron and hole recombination rates along individual structural features
Photoemission Electron Microscopy (PEEM): measure photoemitted electron populations
Ensemble Techniques:
Transient Absorption (TA) Spectroscopy: measure electron and hole recombination rates for a bulk sample