A Novel Ultra‐Sensitive Semiconductor SERS Substrate Boosted by the Coupled Resonance Effect

Abstract Recent achievements in semiconductor surface‐enhanced Raman scattering (SERS) substrates have greatly expanded the application of SERS technique in various fields. However, exploring novel ultra‐sensitive semiconductor SERS materials is a high‐priority task. Here, a new semiconductor SERS‐active substrate, Ta2O5, is developed and an important strategy, the “coupled resonance” effect, is presented, to optimize the SERS performance of semiconductor materials by energy band engineering. The optimized Mo‐doped Ta2O5 substrate exhibits a remarkable SERS sensitivity with an enhancement factor of 2.2 × 107 and a very low detection limit of 9 × 10−9 m for methyl violet (MV) molecules, demonstrating one of the highest sensitivities among those reported for semiconductor SERS substrates. This remarkable enhancement can be attributed to the synergistic resonance enhancement of three components under 532 nm laser excitation: i) MV molecular resonance, ii) photoinduced charge transfer resonance between MV molecules and Ta2O5 nanorods, and iii) electromagnetic enhancement around the “gap” and “tip” of anisotropic Ta2O5 nanorods. Furthermore, it is discovered that the concomitant photoinduced degradation of the probed molecules in the time‐scale of SERS detection is a non‐negligible factor that limits the SERS performance of semiconductors with photocatalytic activity.


Enhancement factor (EF) calculation
The EF of the 15%-Mo-Ta 2 O 5 substrate as the best SERS active substrate was calculated according to the general formula [11] In formula (1) (2), the average number of MV molecules in the scattering region for Raman detection was calculated with the molar mass (M = 408.03 g/mol) and the density (ρ 1.109 g/cm 3 ) of bulk MV. The N A was Avogadro constant, the A spot was the irradiation area of the laser beam with a diameter of 2 μm and the confocal depth h was 21 μm. [12] In formula (3) Thus,

×10 7
Supporting Information 3 In this paper, the first principle calculation method based on density functional theory [13] (DFT) was used to complete the geometric optimization and the electronic structural calculation of Ta 2 O 5 crystal. The periodic boundary condition was used in the calculation process, the local density approximation [14] (LDA) method was applied for the inter-electronic exchange correlation energy, and the ultra-soft potential (Ultrasoft) was used to achieve the interaction potential between ion core and valence electrons. In the wave vector K-space, the cut-off energy of plane wave was chosen as 400 eV, the Brillouin zone (integral = 1 × 4 × 3) was summed according to the special K-point of Monkors-Park. [15] The special K points summed for the Brillouin area. When the total energy change of the system stable within Supporting Information 4

Experimental setup
Our experiment employed a femtosecond CPA laser system that generates laser pulses with 250 kHz repetition rate, 800 nm central wavelength, and 70 fs pulse duration. The generated laser beam was split into two. One beam was frequency doubled by BBO crystal to generate 400 nm laser pulses. The other beam was reflected by a retro-reflector on a motorized linear stage and then focused onto the sample surface as the probe beam. Methyl Violet (MV) crystals were stick on a sample holder. Pristine Ta 2 O 5 powders and powders of Ta 2 O 5 with absorbed MV molecular (noted as MV-Ta 2 O 5 ) were stick on a BK7 glass using black tap and were pressed by another glass to make a flat surface. The power of the pump pulse was 0.1mW, and 0.05mW for probe pulse, the corresponding fluence was 3.18 mJ/cm 2 and 1.59 mJ/cm 2 , respectively.

Results and Discussion
Both pure Ta 2 O 5 and MV-Ta 2 O 5 samples were investigated to reveal the impact of photoinduced decay on the carrier dynamics behaviour in MV-Ta 2 O 5 . We measured the differential reflectivity ΔR/R 0 of the probe beam as a function of the delay time between the pump and probe beams. This reflected the modification of the dielectric constant by the excited state carriers and lattice collective excitations along with the pump beam excitation. [17] In Figure   S11, The experimental data of the ultrafast dynamics can be fitted by the following equation: where A fast and A slow denoted amplitudes, τ fast and τ slow standed for the lifetimes of the excited carriers, A 0 was the background level. With Equation (4)  We furthermore performed a laser power dependence experiment on pure MV sample, for which we illustrated the results in Figure S12. The differential reflectivity of three MV crystals were collected under 0.02 mW, 0.05 mW and 0.1 mW pump powers, respectively. All three tested MV samples were exposed to pump laser beam for 60 minutes. It can be seen that for the higher pump power case, the dynamics curve clearly exhibited an additional faster relaxation component, now with τ fast =0.96ps and τ slow =6ps. For the lower pump power case, there is only one relaxation component, for which the lifetime is much longer, as τ =200ps.
We contemplate there are two channels that contribute to the photo-induced decay of the MV molecule in our experiment on MV-Ta 2 O 5 : (a) the photo-catalytic degradation of MV molecule by Ta 2 O 5 and (b) the direct photo-bleaching for the MV molecule (without Ta 2 O 5 ).
The two factors combine together to yield the overall photo-induced decay. The decay rate for both channels increases with the increasing illumination laser beam power. Considering this, the dynamic under low pump power in Figure S12 (yellow diamonds) is closer to the pure dynamic of MV molecule without photo-induced decay.