High SERS Sensitivity Enabled by Synergistically Enhanced Photoinduced Charge Transfer in Amorphous Nonstoichiometric Semiconducting Films

Abstract

Semiconducting surface‐enhanced Raman scattering (SERS) materials have attracted tremendous attention for their good signal uniformity, chemical stability, and biocompatibility. Here, a new concept to design high sensitivity semiconducting SERS substrates through integration of both amorphous and nonstoichiometric features of WO3−x thin films is presented. The integration of these two features provides narrower bandgap, additional defect levels within the bandgap, stronger exciton resonance, and higher electronic density of states near the Fermi level. These characteristics lead to a synergy to promote the photoinduced charge transfer resonance between analytes and substrate by offering efficient routes of charge escaping and transferring as well as strong vibronic coupling, thus realizing high SERS activity on amorphous nonstoichiometric WO3−x films.

Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

“…Similarly, Fan and coworkers integrated non-stoichiometric and amorphous features in WO 3 −x thin films, which showed an EF up to 10 7 and an LOD of 10 −9 M for R6G. 68 The authors excluded possible EM contributions, as the LSPR of WO 3 −x could hardly be excited at the excitation wavelength of 532 nm. Instead, a promoted PICT resonance was believed to be the reason for the observed SERS enhancement, benefiting from the narrowed band gap, the defect levels generated within the band gap, the strong exciton resonance, and the high electronic DOS near the Fermi level of the WO 3 −x substrate.…”

The origin of ultrasensitive SERS sensing beyond plasmonics

“…In recent years, as the plasmon-free SERS flourish, the researches on its enhancement mechanisms have made remarkable progress. Until now, plenty of studies indicate that plasmon-free SERS strongly rely on material size, surface defect, sample morphology, crystallinity, and crystal orientation [64][65][66][67][68][69][70][71][72]. Some associated phenomena including Mie resonance, CT resonance, exciton resonance and molecular resonance may play important roles solely or synergistically.…”

“…and MV have been widely exploited on the plasmon-free substrates to obtain ultrasensitive SERS sensing by coupling resonance [19,64].…”

“…Recent years, a series of new enhancement strategies including defect engineering [17][18][19], amorphization treatment [64][65][66], phase engineering [25,27], heterojunction engineering [61,67,68], facet engineering [69,70], molecular engineering [46,47], and so on have been used to improve the sensitivity of plasmon-free SERS materials, providing the cases for understanding its enhancement mechanisms. These ultrasensitive plasmon-free SERS substrates expand the applications of plasmon-free SERS into various fields, ranging from bioanalysis to photoelectric characterization.…”

Nucleation and Growth-Controlled Facile Fabrication of Gold Nanoporous Structures for Highly Sensitive Surface-Enhanced Raman Spectroscopy Applications

“…SERS utilizes the plasmonic enhancement of the local electric field (referred to as localized surface plasmonic resonance (LSPR)) that occurs at nanoscale gaps of less than 10 nm, or "hot spots," in various porous structures of gold or silver. These plasmonic nanostructures are known to amplify Raman signals by several orders of magnitude, which enables the probing of tiny amounts of target molecules [8][9][10][11][12][13][14][15][16][17]. The enhancement of the electric field becomes more noticeable as the size of the nano-gaps decreases with increasing density.…”