Surface plasmon resonance (SPR) spectroscopy is a powerful optical technique for probing semiconductor interfaces with high sensitivity to changes in refractive index and charge carrier interactions. It exploits the collective oscillations of free electrons at the interface between a metal and a semiconductor or dielectric, enabling real-time, label-free detection of interfacial phenomena. Two primary modes of SPR exist: localized surface plasmon resonance (LSPR) in nanostructures and propagating surface plasmon resonance (PSPR) in thin films. Both play critical roles in semiconductor research, particularly in sensing and optoelectronic applications.
LSPR occurs in metallic nanostructures such as nanoparticles, where incident light induces coherent electron oscillations confined to the nanoscale dimensions of the structure. The resonance condition depends on particle size, shape, composition, and the surrounding dielectric environment. For semiconductor interfaces, LSPR is often studied in hybrid systems where metal nanoparticles are integrated with semiconductors like silicon, gallium arsenide, or transition metal dichalcogenides. The strong near-field enhancement around these nanoparticles modifies the optical absorption and emission properties of adjacent semiconductors, enabling applications in photodetection and light harvesting. For example, gold nanoparticles on a silicon substrate exhibit LSPR peaks tunable from visible to near-infrared wavelengths based on particle diameter and spacing. The coupling between LSPR and excitons in semiconductors can enhance photoluminescence or facilitate charge transfer, critical for photovoltaic and photocatalytic devices.
PSPR, on the other hand, involves surface plasmon waves propagating along the interface between a thin metal film and a semiconductor. These waves are excited under specific conditions of angle, wavelength, and polarization, typically using prism-coupled configurations like Kretschmann or Otto geometries. The evanescent field of PSPR extends into the semiconductor, making it highly sensitive to interfacial changes such as adsorption, doping, or phase transitions. In semiconductor research, PSPR monitors processes like oxide formation on silicon, molecular adsorption on indium tin oxide, or charge accumulation in organic semiconductors. The resonance angle or wavelength shifts provide quantitative information about thickness or dielectric property changes with sub-nanometer precision. For instance, a 1 nm oxide layer on silicon can induce a measurable shift in the SPR angle, allowing in-situ oxidation kinetics studies.
The applications of SPR spectroscopy in semiconductor interfaces span sensing and optoelectronic coupling. In sensing, LSPR-based platforms detect trace analytes by tracking resonance wavelength shifts due to molecular binding events. Semiconductor nanostructures functionalized with receptors can achieve detection limits in the picomolar range for biomolecules or gases. Gold nanoparticle-decorated zinc oxide nanowires, for example, exhibit LSPR shifts upon exposure to volatile organic compounds, enabling ultrasensitive environmental monitoring. PSPR sensors integrated with semiconductors are used for real-time monitoring of thin-film growth during deposition processes, ensuring optimal quality in photovoltaic or LED manufacturing.
In optoelectronics, SPR enhances light-matter interactions at semiconductor interfaces. LSPR in metal nanoparticles increases absorption in solar cells by scattering light or generating hot electrons that inject into adjacent semiconductors. Plasmonic nanostructures in perovskite solar cells have improved power conversion efficiencies by over 15% through enhanced light trapping. Similarly, PSPR in thin metal films couples with waveguide modes in semiconductor lasers, reducing lasing thresholds or enabling directional emission. Surface plasmons also mediate strong coupling between photons and excitons in two-dimensional semiconductors, leading to hybrid quasiparticles called plexcitons with potential applications in quantum optics.
The choice between LSPR and PSPR depends on the semiconductor system and desired application. LSPR offers simplicity and compatibility with solution-processed semiconductors but lacks the long-range propagation useful for integrated photonic circuits. PSPR provides higher sensitivity and better compatibility with planar device architectures but requires precise optical alignment. Advances in nanofabrication have enabled hybrid approaches, such as grating-coupled PSPR with embedded nanoparticles, combining the benefits of both modes.
Challenges remain in optimizing plasmonic-semiconductor interfaces. Ohmic losses in metals limit the resonance quality factor, while uncontrolled charge transfer can quench desired optical effects. Recent work focuses on alternative plasmonic materials like transparent conducting oxides or highly doped semiconductors, which exhibit lower losses and better band alignment with conventional semiconductors. Another area of progress is the dynamic tuning of SPR via external stimuli such as electric fields or strain, enabling reconfigurable optoelectronic devices.
In summary, SPR spectroscopy provides unique insights into semiconductor interfaces through LSPR and PSPR, driving advancements in sensing and optoelectronics. Its ability to probe interfacial dynamics with high spatial and temporal resolution makes it indispensable for developing next-generation semiconductor devices. Continued innovation in materials and coupling strategies will further expand its utility in emerging technologies like quantum photonics and biointegrated electronics.