In this work, we study the charge transfer improvement by the combination of two semiconductors of SERS. The energy levels of the semiconductor, when combined, become intermediate energy levels that support the charge transfer from the HOMO to the LUMO level, amplifying the Raman signal of the organic molecules. The SERS substrates of Ag/a-Al₂O₃–Al/ZnO nanorods with high sensitivity are prepared for detecting dye rhodamine 6G (R6G) and metronidazole (MNZ) standard. The highly ordered vertically grown ZnO nanorods (NRs) are first developed on a glass substrate by a wet chemical bath deposition method. Then, ZnO NRs are covered with an amorphous oxidized aluminum thin film by a vacuum thermal evaporation method to produce a platform with a large surface area and high charge transfer performance. Finally, silver nanoparticles (NPs) are decorated onto this platform to form an active SERS substrate. The structure, surface morphology, optical properties, and elements in the sample are investigated by Raman spectroscopy, X-ray diffractometry, field-emission scanning electron microscopy (FE-SEM), ultraviolet-visible spectroscopy (UV-vis), reflectance spectroscopy, and energy dispersion X-ray spectroscopy (EDS). Rhodamine 6G is used as a reagent to evaluate the SERS substrates with an analytical enhancement factor (EF) of ∼1.85 × 10¹⁰ at the limit of detection (LOD) of 10⁻¹¹ M. These SERS substrates are used to detect metronidazole standard at a LOD of 0.01 ppm and an EF of 2.2 × 10⁶. The SERS substrate exhibits high sensitivity and stability for promising wide application in chemical, biomedical, and pharmaceutical detection.

We fabricated surface-enhanced Raman scattering (SERS) substrates using gold nanoparticle (AuNP)-decorated zinc oxide (ZnO) nanorods (NRs). Prior to decoration with AuNPs, ZnO NRs on the glass substrate fabricated using the sol–gel method could enhance the SERS signal for detecting 10⁻⁵ M rhodamine 6G (R6G). Microscopic analysis revealed that the thermal-annealing process for fabricating the seed layers of ZnO facilitated the growth of ZnO NRs with the highly preferred c-axis (002) orientation. A decrease in the diameter of ZnO NRs occurred because of the use of annealed seek layers further increased the surface-to-volume ratio of ZnO NRs, resulting in an increase in the SERS signal for R6G of 10⁻⁵ M. To combine the localized surface plasmon resonance (LSPR) mode with the charge transfer (CT) mode, ZnO NRs were decorated with AuNPs through pulsed-laser-induced photolysis (PLIP). However, the preferred vertical (002) orientation of ZnO NRs was prone to the aggregation of AuNPs, which hindered the SERS signal. The experimental results revealed that ZnO NRs with the crystalline structure of horizontal (100) and (101) orientations facilitated the growth of homogeneous, independent and isolated AuNPs which serves as “hot spots” for SERS signal of detecting R6G at a low concentration of 10⁻⁹ M. Comparing to previous fabrication of SERS substrate, our method has advantage to fabricate AuNP-decorated ZnO NR in a short time. Moreover, the optimization of the SERS behaviors for different fabrication conditions of AuNPs using the PLIP method was investigated in detail.

In Surface-Enhanced Raman Scattering (SERS) analysis, enhancing the Raman signal is crucial and is primarily governed by two mechanisms: electromagnetic mechanism (EM) and chemical mechanism (CM). While the EM is well understood, gaining insight into the CM and its synergy with EM is crucial for optimizing SERS performance. In this study, CM effects were investigated through molecular adsorption and charge transfer (CT) processes using SERS substrates composed of silver nanoparticles (AgNPs) integrated with aluminium-doped zinc oxide (AZO) semiconductor thin films. AgNPs were synthesized via microwave irradiation and centrifugation, followed by incubation with Rhodamine 6G (R6G) and Methylparaben (MP) to enhance analyte adsorption. Adsorption behaviours were analysed using the Langmuir and Freundlich isotherm models. The analyte-loaded AgNPs were then combined with AZO thin films prepared by sputtering. These hybrid substrates not only generated electromagnetic “hot spots” to boost Raman signals via the EM but also facilitated CT, thereby enhancing signals through CM. A proposed CT mechanism between the substrate and analytes is also presented. The results demonstrate that the Ag/AZO SERS substrates exhibit a strong synergistic EM–CM effect, enabling the sensitive detection of R6G and MP at low concentrations, achieving enhancement factors (EFs) of 8.67 × 10¹⁰ (at 10⁻¹³ M) and 1.45 × 10⁶ (at 0.001 ppm or 6.58 × 10⁻⁹ M), respectively.

In this study, two layers: i-ZnO nanorods and p-Cu2O were fabricated by electrochemical deposition. The fabricating process was the initial formation of ZnO nanorods layer on the n-IGZO thin film which was prepared by sputtering method, then a p-Cu2O layer was deposited on top of rods to form the p-Cu2O/i-ZnO nanorods/n-ZnO heterojunction. The XRD, SEM, UV–VIS, I–V characteristics methods were used to define structure, optical and electrical properties of these heterojunction layers. The fabricating conditions and thickness of the Cu2O layers significantly affected to the formation, microstructure, electrical and optical properties of the junction. The length of i-ZnO nanorods layer in the structure of the heterojunction has strongly affected to the carriers transport mechanism and performance of this heterojunction.

In recent years, surface-enhanced Raman scattering (SERS) based on metal oxide semiconductors has been an active area of research and development, attracting significant scientific interest. These SERS substrates are known as plasmon-free SERS substrates because they are not based on noble metal nanoparticles but mainly on the defects, structure, and surface morphology of semiconductors to enhance the Raman signal. In this study, we fabricated a SERS substrate based on molybdenum oxide, using reactive DC magnetron sputtering and then used different simple and effective strategies to enhance the Raman signal. The results show that nanorod structure, oxygen deficiency engineering, phase engineering, and optical properties can be easily controlled by varying sputtering time and annealing time of MoO_x SERS substrates. The analysis methods XRD, PL, and Raman show that with the optimal fabricated conditions. The presence of oxygen defects and a mixed MoO₃, Mo₉O₂₆ phase structure in as well as the nanorod structure of MoO_x SERS substrates could likely enhance Raman signals via a chemical mechanism (CM) and electromagnetic mechanism (EM). The MoO_x SERS substrates were also used to detect R6G at low concentrations, with an EF of 1.14 × 10⁶ (at 0.01 ppm), LOD of 0.01 ppm, and good temporal stability and reproducibility.