Micro Arrays Of Precious Metal/Mo-based Semiconductor Nanoparticles And Their Surface-enhanced Raman Scattering (SERS) Studies

Raman spectroscopy is a scattering spectrum generated by inelastic collisions of incident light,which has the characteristics fingerprints of molecularand can be used for biochemical specific detection.Surface Enhanced Raman Scattering（SERS）technology combines Raman spectroscopy and nanotechnology,which greatly enhances the signal intensity of Raman spectroscopy and has the advantages of high sensitivity,easy operation and non-destructive.It has been widely used in important areas,such as environmental pollution monitoring,food quality monitoring,biomedical analysis,etc.The key factor in the development of SERS technology lies in the development of high-performance enhanced substrates.The key factor in developing SERS technology is the development of high-performance enhancement substrates.Precious metal materials,especially Au and Ag nanomaterials,have become the most widely used substrates with the strongest SERS effect due to their localized surface plasmon resonance（LSPR）effect.However,the application cost of Au nanomaterials is high,and the stability of Ag nanomaterials still needs to be improved.For these reasons,the development of a low-cost and highly stable SERS substrate with enhancement capabilities comparable to precious metals such as Au and Ag has become a key challenge for the advancement of SERS technology.In this paper,we focus on the preparation,performance and enhancement mechanism of noble metal/semiconductor composite SERS substrates,which mainly includes the following aspects:（1）A unique three-step method:femtosecond laser processing,magnetron sputtering coating and controlled heat treatment were used to efficiently and environmental friendly fabricate SERS substrates with excellent sensitivity,high homogeneity and high stability of noble metal semiconductor Si composite micro and nano array structure.The average width of the nanogrooves of the Si substrate is about 112.42 nm,and the average particle size of the nanobumps is about 98.06 nm.The self-assembled Au nanoparticle arrays with the strongest Raman signal was found in sapmple with film thickness of 26 nm,which was obtained by controlling the magnetron sputtering and heat treatment conditions at a 600°C for120 min.This efficient and controllable self-assembly of Au nanoparticles can be a reference for the self-assembly of metal nanoparticles on other hard substrates.（2）The prepared Au/Si composite structured SERS active substrates were tested for SERS performance.Rhodamine 6G（R6G）was used as the detection molecule,and the characteristic peak of R6G at 611 cm^-1 was taken for calculation.An enhancement factor（AEF）of 1.7×10⁴ and a remarkably low limit of detection（LOD）of 10^-9 M were achieved,which is one of the best SERS performance found in metal/semiconductor composite structures,demonstrating superior SERS performance for metal/semiconductor composite structures.This low LOD is comparable to those achieved with precious metals,placing the composite structures in the highest echelon for sensitivity.The uniformity test shows that the standard deviation of the signal at 60 different positions of the substrate is as low as 2.74%,and the stability test with continuous measurements at 30 days interval shows only about 4.3%attenuation of the signal intensity.This indicates that our substrates have excellent uniformity and stability.Simulations utilizing the finite-difference time-domain algorithm（FDTD）helped to visualize the substrate enhancement mechanism,which indicated that the electromagnetic coupling between multimers like dimer and trimer with Au nanoparticles and between Au nanoparticles and the inner wall of the nanotrench were responsible for the local surface plasmon resonance.（3）The unique one-step method was used to prepare largeα-Mo O₃ single crystals in high quality single,and the prepared single crystal is one of the largest size have been reported.The analysis of the material shows that the single crystal has good crystallinity and high purity.The largest size and maximum output ofα-Mo O₃ single crystal clusters were obtained in26tube diameter with a growth time of 30 min at 750°C.（4）We innovatively use femtosecond laser to treat large-sizedα-Mo O₃ single crystals,which can efficiently and rapidly transformα-Mo O₃ from non-SERS active substrates to SERS active substrates,greatly expanding the possibilities ofα-Mo O₃ in SERS applications and provided a new idea to effective transformation of non-SERS active semiconductor substrates.After laser processing of Mo O₃ single crystals,XPS measurement reveals the Mo5+in the 3d orbitals and O1s orbitals at 531.9 e V.The new peak at 531.9 e V can be attributed to the oxygen defect,while the XRD results show no change in the peak position before and after laser processing,which can confirm that the oxygen defect was successfully modulated without changing the structure of material.The SERS performance of Mo O₃-NSs substrates were tested with R6G molecules,and the results showed that the substrates selectively enhanced the characteristic peaks at different positions,and the AEFs of 0.3×10⁴ and 0.5×10⁴ were calculated with two characteristic peaks at 611 cm^-1 and 1361 cm^-1 respectively,which also confirmed the existence of the chemical enhancement（CM）mechanism from the side.The LOD was as small as10^-7M.Repeatability tests at one-month intervals showed almost no signal attenuation,indicating excellent sensitivity and stability of the substrate.The uniformity test showed a relative standard deviation of 8.56%for 36 different positions on the substrate.The large relative standard deviation of the substrate attributes to the fact that the connected portions of the adjacent perimeters of the micro nanostage were not fully scanned by the laser,showing a relatively low SERS enhancement.We attribute the Raman enhancement of the substrate to the enhanced molecular polarization induced by photo-induced charge transfer（PICT）and the synergistic enhancement of the strong LSPR effect of the quantum dots on Mo O₃.（5）Further,we fabricated Mo O₃-NSs composite substrates with metallic Au,and the newly fabricated composite substrates show three orders of magnitude performance enhancement over the original substrates,with AEFs as high as 1.0×10⁶ and LODs as low as 10^-9M,which is one of the best performance have been reported of precious metal semiconductor composite substrates.We attribute the additional Raman signal enhancement to the plasmon resonance-induced electromagnetic enhancement（EM）of Au itself and to the PICT-dominated chemical enhancement between R6G and Au@Mo O₃-NSs.