Nanoscale materials demonstrate unique properties highlighted by the tunability of their electronic and optical behavior, chemical reactivity, and mechanical or structural stability. The resultant features make nanomaterials attractive for diverse applications in sensing, imaging, and catalysis, among others. However, the very advantages that make them important also make them difficult to characterize.
A variety of techniques have been employed to study nanomaterials. The relevant science boils down to either a spectroscopic or microscopic analysis involving nanomaterials interacting with electromagnetic radiation (light) or electrons, respectively. These interactions subsequently provide valuable information about the nanomaterials’ optical properties, morphology, composition, and crystal structure. In consideration of the wide array of approaches available in the field, this article will focus on the broader spectroscopic techniques and principles that facilitate nanomaterial characterization.
Optical spectroscopy considers the interaction of matter with light spanning a wide spectrum from long-wavelength, low energy radio waves, to short-wavelength, high energy gamma rays. The nature of light-matter interactions depends on the wavelength and intensity of light, and its influence on the molecules or atoms of the sample. When light strikes a sample, one of three things can happen—scattering, absorption, or emission—corresponding to transitions in the energy state of the sample’s molecules, atoms, or ions. This information is contained in the sample’s reflectance, transmittance, and absorbance spectra, and all are measured using a spectrometer.
A wide variety of optical spectroscopic techniques are available to characterize nanomaterials, including ultraviolet-visible-near infrared (UV-Vis-NIR), photoluminescence (PL), Fourier-transform infrared (FTIR), and Raman spectroscopy. UV-Vis-NIR spectroscopy, covering the ultraviolet-visible-near infrared spectrum, is the study of electronic absorption where the resulting optical spectra detail the electronic structure, focusing on the electronically excited states and phonon replicas of the given sample. UV-Vis-NIR spectroscopy offers a facile means to optically characterize plasmonic metal nanoparticles as it can directly measure their spectral extinction behavior. This assists in determining the relevant wavelengths corresponding to the plasmon resonances—collective electron charge oscillations in metal nanoparticles excited by light and resulting in enhanced near-field absorption at the resonance wavelength. Measuring the linewidths of the plasmon resonance peaks using UV-Vis-NIR spectra can then provide information on local variations in permittivity of the host particle, non-uniformity in nanoparticle size, nanoparticle coupling effects and polydispersity, aggregation phenomena, as well as nanoparticle morphology and geometry.
The electronic excitation of the sample to a higher energy state is followed by a relaxation process, termed photoluminescence, involving the spontaneous emission of a photon. PL spectroscopy is effective in determining the electronic and vibrational structures of a molecule and probing radiative and non-radiative relaxation channels as well as their corresponding timescales. PL spectroscopy can discern the optoelectronic properties of materials sized between nanometers to centimeters and is commonly utilized to characterize semiconductors and molecules. PL spectroscopy provides a non-destructive means to determine the electronic bandgap of a semiconductor, helping quantify its elemental composition and its efficiency for use as a solar cell device. PL peaks associated with radiative transitions in semiconductors are also indicative of localized defects and their corresponding concentrations. Analysis of PL peak intensities and lifetimes involving radiative transitions can also be used to characterize biological samples coated with fluorescent molecules that enable tracking and sensing of biomolecules.
FTIR can be used to determine a sample’s molecular composition and structure by analyzing its absorbance of infrared light energy at various wavelengths. Raman spectroscopy uses the same spectral range but measures the amount of light scattered by the sample to examine its vibrational and rotational modes. Surface-enhanced Raman spectroscopy considers the amplification of Raman signals from molecules adsorbed on metallic or semiconductor surfaces. These adsorbed molecules serve as probes that characterize interfacial electronic and vibrational transitions that are essential for photocatalytic applications such as dye-sensitized solar cells. In conjunction with FTIR, the two techniques are especially useful in measuring diverse characteristics of nanomaterial samples including the concentration of chemicals, surface chemistry, functional groups, and atomic arrangements of biological adsorbates.
The optical properties of a nanomaterial are heavily influenced by its morphology. Electron microscopy methods have replaced traditional optical microscopes and visualize the nano regime by utilizing electron beams to create images highlighting the morphologies of nanomaterials. Electron microscopy overcomes the diffraction limit that optical microscopy techniques suffer by using electrons, which have a far smaller wavelength than that of visible light photons, for a thousandfold improvement in resolution.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are among the most popular electron microscopy techniques used today to directly visualize nanomaterials. The two methods are distinguished by how their images are created; SEM uses reflected or scattered electrons and TEM uses transmitted electrons that pass through the sample. As such, SEM provides information on the sample’s surface morphology and composition while TEM offers information on the inner structure of the sample such as its crystal structure, morphology, and stress state. In addition, SEM and TEM have different spatial resolution extremes. SEM resolution is limited to approximately 0.5 nm while TEM can image at spatial resolutions less than 50 pm.
The coupling of electron microscopy with spectroscopic techniques enables the elemental and structural characterization of nanomaterials. This involves analyzing the bulk chemical composition of a nanomaterial, its atomic constituents, or the crystal structure of the nanomaterial. The crystal structure of a material describes the nature of its atomic arrangement. By measuring the crystallinity of a material and obtaining information on interfacial atomic arrangements, inferences on the material’s physiochemical properties can be determined. Ensemble techniques such as X-ray diffraction (XRD) can be paired with single-particle techniques like dispersive X-ray spectroscopy (EDX) for elemental and structural studies of nanomaterials.
XRD provides details about a material’s crystalline phase, particle size, and the bonds between its atoms and bulk features. The characteristic diffraction patterns of XRD are representative of the physiochemical and structural properties of the material, yielding information on structural deviations due to internal stresses and defects. XRD on thin metallic films can provide information on their metallicity, surface and lattice defects, grain boundaries, and crystal structure, via measurements of crystallite sizes and microstrains. This can subsequently determine the adsorption characteristics of thin films on various surfaces which is fundamental to applications including microelectronics, antireflective coatings, sensors, solar panels, etc.
Similarly, EDX relies on the emission of characteristic x-rays from a material specimen stimulated by a high-energy beam of charged particles, such as electrons, for elemental analysis or chemical characterization. This is particularly useful for analyzing thicknesses of multi-layered metal films and the compositional ratios of multi-metallic nanomaterials. In the latter, the ability to elementally map the composition of nanostructures can shed light on the morphological characteristics of alloys, bimetallic core-shell, intermetallic, and sub-cluster architectures. As electron beam excitation is often used in SEM and TEM systems, most electron microscopy systems accompany an EDX setup allowing for morphological and elemental characterization.
Characterizing nanomaterials—an emerging field of research
Nanoscience is a growing field that has infiltrated various disciplines of science and engineering. Beyond UV-Vis-NIR, FTIR, SEM, TEM, XRD, and EDX, many other techniques have been employed for nanomaterial characterization. In fact, nanomaterial characterization is itself an emerging area of research posing several challenges. It is not an easy task to bring to light what happens in the nanoworld, but the right characterization technique can help optimize existing nanofabrication methods, technology, and provide for greater advances in nano-research where the physics behind these phenomena can be unraveled.