[Paper Review] Chemical analysis of ligand-free silicon nanocrystal surfaces by surface enhanced Raman spectroscopy

İlker Doğan, Ryan Gresback+2 more|arXiv (Cornell University)|Dec 30, 2014

Gold and Silver Nanoparticles Synthesis and Applications7 citations

TL;DR

This study demonstrates that surface-enhanced Raman spectroscopy (SERS) using Ag/AgₓO substrates enables real-time, non-destructive monitoring of ligand-free silicon nanocrystal (Si-NC) surface chemistry under air-free conditions. It reveals dynamic oxidation processes, including the gradual disappearance of Si–Clₓ and Si–Hₓ modes and the emergence of Si–Oₓ and SiOH modes, with oxide thickness reaching ~1 nm after 90 minutes of air exposure, which decouples plasmonic enhancement and terminates SERS signals.

ABSTRACT

Surface enhanced Raman spectroscopy (SERS) was used to probe the surface chemistry of chlorine-terminated silicon nanocrystal (Si-NC) surfaces in an air-free environment. SERS effect was observed from the thin films of Ag$_x$O using 514 nm laser wavelength. When a monolayer of Si-NCs were spin-coated on Ag$_x$O SERS substrates, a very clear signal of surface states, including Si-Cl$_x$, and Si-H$_x$ were observed. Upon air-exposure, we observed the temporal reduction of Si-Cl$_x$ peak intensity, and a development of oxidation-related peak intensities, like Si-O$_x$ and Si-O-H$_x$. In addition, first, second and third order transverse optical (TO) modes of Si-NCs were also observed at 519, 1000 and 1600 cm$^{-1}$, respectively. As a comparison, Raman analysis of a thick film (> 200 nm) of Si-NCs deposited on ordinary glass substrates were performed. This analysis only demonstrated the first TO mode of Si-NCs, and the all the other features originated from SERS enhancement did not appear in the spectrum. These results conclude that, SERS is not only capable of single-molecule detection, but also a powerful technique for monitoring the surface chemistry of nanoparticles.

Motivation & Objective

• To develop a non-destructive method for monitoring the surface chemistry of ligand-free silicon nanocrystals (Si-NCs) in real time.
• To investigate the dynamic surface passivation and oxidation processes of chlorine-terminated Si-NCs upon air exposure.
• To evaluate the capability of surface-enhanced Raman spectroscopy (SERS) in detecting surface-specific vibrational modes of Si-NCs, including Si–Clₓ, Si–Hₓ, and oxidation-related species.
• To determine the role of plasmonic enhancement in selectively probing surface chemistry versus bulk Si-NCs.
• To correlate SERS signal evolution with oxide layer thickness on Si-NCs, particularly the impact of increasing distance on signal quenching.

Proposed method

• SERS measurements were performed using thin films of Ag/AgₓO as plasmonic substrates under an oxygen-free environment.
• Monolayers of chlorine-terminated Si-NCs were spin-coated onto Ag/AgₓO substrates to enable surface-specific signal enhancement.
• 514 nm laser excitation was used to induce surface plasmon resonance and enhance Raman scattering from surface-adsorbed species.
• Time-resolved SERS spectra were collected after controlled air exposure (10 min, 90 min) to monitor surface chemistry evolution.
• Raman spectra were compared with conventional Raman on thick Si-NC films on glass to isolate SERS-specific contributions.
• Peak assignments were based on reference data for Si–Clₓ, Si–Hₓ, Si–Oₓ, and SiOH modes, with emphasis on distinguishing surface from bulk signals via distance-dependent enhancement (d⁻¹⁰ scaling).

Experimental results

Research questions

• RQ1 Can SERS detect and monitor surface-specific vibrational modes of ligand-free Si-NCs with high sensitivity and selectivity?
• RQ2 How does air exposure affect the surface chemistry of chlorine-terminated Si-NCs, as revealed by SERS?
• RQ3 What is the relationship between oxide layer thickness on Si-NCs and the quenching of SERS signals?
• RQ4 To what extent can SERS distinguish surface-adsorbed species (e.g., Si–Clₓ, Si–Hₓ) from bulk Si-NC vibrational modes?
• RQ5 Does the SERS enhancement effect remain effective for Si-NCs after the formation of a 1 nm oxide shell?

Key findings

• SERS detected distinct surface modes of Si-NCs, including Si–Clₓ at 450, 540, and 650 cm⁻¹, Si–Hₓ wagging at 600–630 cm⁻¹, and Si–Hₓ stretching at 1900–2200 cm⁻¹, confirming surface-specific chemistry.
• After 10 minutes of air exposure, Si–Clₓ and benzonitrile modes disappeared, while Si–OH at 2920 cm⁻¹ and SiOSiH at 800 cm⁻¹ increased, indicating early oxidation.
• After 90 minutes of air exposure, the main Si–NC vibrational modes vanished, and the spectrum was dominated by oxidation-related peaks: SiOSi at 450 and 1100 cm⁻¹, SiOSiH at 800 cm⁻¹, and SiOH at 900 cm⁻¹.
• The SERS signal quenching was attributed to a 1 nm oxide layer on Si-NCs, which increased the distance between the Si-NC surface and the LSPR zone beyond effective enhancement range (d⁻¹⁰ dependence).
• The Si–Hₓ stretching modes at 1900–2100 cm⁻¹ remained detectable after oxidation, indicating selective surface sensitivity of SERS to surface-adsorbed species.
• Conventional Raman on thick Si-NC films on glass only detected the first-order TO mode at 519 cm⁻¹, confirming that other features were SERS-specific and surface-selective.