Raman spectroscopy is a useful technique that can disclose the atomic structural aspects of nanomaterials and their electrochemical properties. The study of semiconducting nanocrystals has significantly benefited from Raman spectroscopy.
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What are Semiconductor Nanocrystals?
Semiconductor nanocrystals are conducting materials with a size that has been reduced to the nanoscale. This size reduction can greatly enhance the physical, chemical, and optical characteristics, such as absorption coefficient and refractive index, of these nanoparticles.
During the last decade, the enhancement of quantum size effects, nonlinear optical characteristics, and luminescence have been studied in semiconductor nanocrystals.
Significance of Raman Spectroscopy
Semiconductor nanoparticles affect electronic states by spatially confining excited electron-hole pairs in a volume smaller than the symmetric exciton Bohr radius.
Raman spectroscopy is a potential approach for studying these phenomena, notably the influence of electrical and atomic structures on each other. It is a spectroscopic method that can detect rotational and low-frequency modes in systems. The crystallographic and electrical environments may affect the method's detection of lattice vibrations.
Raman spectroscopy is widely used in materials science to identify compounds by their structural fingerprint. In Raman spectroscopy, the light source is usually a laser. Laser light is used because it emits a highly powerful, almost monochromatic beam of light that can interact with sample molecules. When matter absorbs light, it changes its fundamental energy in some manner.
Raman Spectroscopy to Analyse Semiconductor Nanocrystals
Nanocrystals made of heterogeneous chemical domains exhibit various features that make them attractive for integration into next-generation devices, light sources, and bioelectronics. However, the exact spatial distribution of the materials contained inside these nanoparticles is difficult to quantify and regulate, even though it significantly influences their reliability and effectiveness.
The Raman spectroscopy technique enables researchers to analyze a wide variety of nanostructures, their structural and chemical compositions, physicochemical characteristics, and interactions with other particles. Raman spectra can be used to effectively categorize their internal structures.
These classifications give direct information on the elemental composition and an independent forecast of the fluorescence emission yield. This non-destructive, fast technique has the potential to significantly improve the capability for measuring, forecasting, and monitoring multicomponent nanomaterials to tune their structures and characteristics precisely.
In semiconductor nanocrystals, internal structure assessment, the subtle difference between an alloy and a (core) shell material should manifest itself in a redistribution of bond strain and electronic polarization between different atoms. Raman spectroscopy can be used to accurately predict the internal structure and optical properties of nanocrystal heterostructures.
Optical and electronic properties play a vital role in semiconductor nanocrystals. A small change in optoelectronic properties of semiconducting nanocrystals can dramatically alter their quantum yield (QY), electron transfer rate, and device performance. Raman spectroscopy can be used to predict the changes by providing information about tuning size, the composition of nanocrystals, and their internal structure.
Types of Raman Spectroscopy Used for Analysis of Nanocrystals
There are many types of Raman spectroscopy, such as spatially offset Raman spectroscopy (SORS), tip-enhanced Raman spectroscopy (TERS), and surface-enhanced resonance Raman scattering (SERRS).
SERRS has gained popularity in recent years due to its great sensitivity and selectivity. It enables non-invasive in situ detection of target molecules and improves the Raman scattering of molecules assisted by nanostructured materials.
SORS is a Raman spectroscopy variant that allows for the highly accurate chemical study of substances beneath obscuring surfaces such as tissue, coatings, and bottles. TERS, like conventional far-field Raman spectroscopy, provides chemical information about a nanomaterial.
A Raman spectrum is a unique chemical fingerprint based on the vibrational characteristics of a certain molecule or substance that may be used to quickly identify or distinguish the material from others. It provides information on the chemical structure, defect density, phase and polymorphism, inherent stress/strain, contamination and impurity, molecule orientation, and grafting quality.
Benefits of Raman Spectroscopy in Semiconductor Nanocrystal Analysis
There are several benefits of utilizing Raman spectroscopy in semiconductor nanocrystal analysis.
Raman spectroscopy allows examining the phases and phasing transitions in the structure of the material. It also provides the opportunity to inspect the effect of temperature on phase transitions and the material’s structure. Apart from these advantages, Raman spectroscopy can also monitor numerous variables simultaneously with great precision, allowing for real-time optimization.
Raman spectroscopy enables inspection of structure and quality during semiconductor nanocrystal fabrication, enabling clear structural verification without the need for intensive numerical simulations. In semiconductor nanocrystals, Raman spectroscopy is crucial for determining characteristics such as QY, which is influenced by surface effects in the majority of quantum dots.
Commercial Equipment Available for Raman Spectroscopy
There are several commercial types of equipment readily available for Raman spectroscopy in the market. One example is the LabRAM HR Evolution Raman microscope, suitable for micro and macro quantities, as well as 2D and 3D confocal imaging. With this confocal Raman microscope, comprehensive pictures and data may be produced quickly and confidently.
The measuring wavelength range may be enlarged from 200 nm to 2200 nm due to compatibility with a wide variety of laser wavelengths and the ability to attach up to three detectors. The optimized UV setup is an effective option for UV Raman analysis at wavelengths less than 400 nm.
This can also undertake other spectroscopic processes, such as UV Raman, resonance Raman, and photoluminescence, allowing for comprehensive sample characterization over a wide range of materials.
References and Further Reading
Boldt, K. (2022). Raman spectroscopy of colloidal semiconductor nanocrystals. Nano Futures, 6. Available at: https://doi.org/10.1088/2399-1984/ac4e77
Fasolato, C. Z. (2020). Addressing Crystal Structure in Semiconductor Nanowires by Polarized Raman Spectroscopy. In C. Z. Fasolato, Fundamental Properties of Semiconductor Nanowires. Springer. Available at: https://doi.org/10.1007/978-981-15-9050-4_7
Mukherjee, P., Lim, S. J., Wrobel, T. P., Bhargava, R., & Smith, A. M. (2016). Measuring and Predicting the Internal Structure of Semiconductor Nanocrystals through Raman Spectroscopy. Journal of the American Chemical Society, 10887-10896. Available at: https://pubs.acs.org/doi/10.1021/jacs.6b03907
Yi Cao, M. S. (2022). Tip-enhanced Raman spectroscopy. Reviews in Physics. Available at: https://doi.org/10.1016/j.revip.2022.100067