⇦Chinese Physics Letters, 2020, Vol. 37, No. 6, Article code 068102 Gold-Nanoparticles/Boron-Doped-Diamond Composites as Surface-Enhanced Raman Scattering Substrates * Ai-Qi Zhang (张爱琪), Qi-Liang Wang (王启亮), Ying Gao (高莹), Shao-Heng Cheng (成绍恒)**, Hong-Dong Li (李红东) Affiliations State Key Lab of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China Received 24 December 2019, online 26 May 2020 *Supported by the National Natural Science Foundation of China (Grant Nos. 51672102 and 51972135).
**Corresponding author. Email: chengshaoheng@jlu.edu.cn Citation Text: Zhang A Q, Wang Q L, Gao Y, Cheng S H and Li H D et al 2020 Chin. Phys. Lett. 37 068102    Abstract By vacuum sputtering and annealing processes of gold (Au) films on boron-doped diamond (BDD) surfaces, Au-nanoparticles/BDD (AuNP/BDD) composite substrates were prepared as surface-enhanced Raman scattering (SERS) substrates. The SERS performances of the substrate were investigated using methylene blue molecule as a probe. With the AuNPs having an average diameter of 20 nm, high performance of SERS was achieved at an enhancement factor of $9\times 10^{5}$, arising from the synergistic effect of electromagnetic enhancement from AuNPs and chemical enhancement from diamond. The AuNP/BDD substrate is demonstrated to be highly sensitive, reproducible, stable, and reusable for the SERS examination. Due to the facile preparation process and controllable surface morphology, the AuNP/BDD substrates are favorable as a high performance SERS platform performed in practical applications. DOI:10.1088/0256-307X/37/6/068102 PACS:81.05.ug, 78.30.Am, 81.07.-b © 2020 Chinese Physics Society Article Text Due to the high sensitivity and non-destructive measurements of retrieving chemical and biological information, surface-enhanced Raman scattering (SERS) is highly desirable in many fields, such as medical diagnostics, environmental protection, and food safety.[1,2] Since discovered in the mid-1970s, SERS has attracted continuous attention, and the researches on SERS-active platforms mainly focused on noble metals (e.g., Au and Ag) and various transition metals.[3] Metal nanoparticles (NPs) are advanced over bulk counterparts because of their high enhancement capability.[4] However, there are some disadvantages of metal NPs related to structural uncontrollability, poor biocompatibility, and limited formation of hot-spots in detection zones, which would lead to discrepancies in SERS signals.[5] Recently, charge transition based semiconductor SERS has been a hot topic, due to its attractive inherent properties of adjustable band gaps, stable production of excitons, controllable photoelectrical properties, and long-term stability, with potential for a wide range of applications.[1,6,7] However, semiconductor-based SERS substrates have one main drawback of weak SERS enhancements compared with the enhancements of noble metals.[8,9] In order to increase SERS enhancements, one effective method proposed is to combine a semiconductor material with metal NPs that possess plasmon enhancement in addition to charge transition. Several kinds of metal-semiconductor nanocomposites have been investigated, showing highly efficient SERS activities and facilitating practical applications.[1,7,10] Diamond is a wide bandgap semiconductor, being chemically inert and possessing excellent physical properties such as outstanding thermal stability, super hardness, and high thermal conductivity, leading itself to applications in numerous fields.[11] Recently, BDD film has been demonstrated to be a semiconductor-based SERS substrate material with high-performances,[12] having comparable enhancement factor (EF) to those of typical metal-oxide and metal non-oxide SERS substrates.[13,14] Therefore, a combination of AuNPs and BDD forming an AuNP/BDD substrate would be expected to further enhance the SERS signal. In this Letter, we fabricated a SERS substrate of AuNP/BDD consisting of combined AuNPs and BDD film, showing high performance SERS examinations. The sensitivity, reproducibility, stability, and recyclability were tested, indicating a high potential for practical applications. BDD films were synthesized on p-type (100) silicon (Si) substrates using a microwave plasma chemical vapor deposition system (MPCVD, 2 kW at 2.45 GHz). Before deposition, the Si substrates were mechanically polished with diamond nanopowders and ultrasonically cleaned in deionized water. Hydrogen and methane were used as gas sources with flow rates of 200 sccm and 3 sccm, respectively. The boron source was introduced by bubbling H$_{2}$ gas (flow rate: 6 sccm) through liquid trimethyl borate (B(OCH$_{3})_{3}$) (ambient temperature maintained at 25 $^{\circ}\!$C). The total pressure and deposition time were 8.5 kPa and 12 h, respectively. The deposited BDD films were coated by an Au layer with nanometer thickness by ion sputtering and subsequently, the products were annealed in a tube furnace at 800 $^{\circ}\!$C for 2 min in air to form AuNPs on BDD film.[15] The size of the AuNPs was controlled by varying deposition time of Au films. The morphology of the AuNP/BDD substrates was characterized by scanning electron microscopy (SEM, FEI MAGELLAN-400). SERS capability was evaluated using methylene blue (MB) molecules as a probe. For SERS examinations, a confocal Raman system (LabRAM Aramis, Horiba Jobin Yvon), with 633 nm excitation from a He–Ne laser was used to record the Raman scattering signals. Figures 1(a)–1(d) show the SEM images of the BDD films and AuNP/BDD substrates. As shown in Fig. 1(a), the polycrystalline BDD film consists of grains with an average size of about 5 µm, forming a rough surface. Figures 1(b), 1(c) and 1(d) present annealed AuNPs on the BDD surfaces for the cases of sputtering gold films for 10 s, 20 s and 40 s, respectively, and the corresponding average diameters of the hemispherical AuNPs are about 10 nm, 20 nm, and 30 nm in turn. With increasing the depositing time of the Au film, i.e, increasing the film thickness, the average size of the AuNPs after dewettability process increases gradually. Here the AuNP/BDD SERS substrates are named as S10, S20, and S30, corresponding to the substrates having AuNPs with average diameters of 10, 20, and 30 nm, respectively.
cpl-37-6-068102-fig1.png
Fig. 1. SEM images of as-grown BDD films (a) and AuNP/BDD substrates with the deposited Au layer for 10 s (b), 20 s (c), and 40 s (d). The inset in (c) is the enlarged picture under high magnification.
cpl-37-6-068102-fig2.png
Fig. 2. Raman spectra of MB molecules on AuNP/BDD SERS substrates with various sizes of AuNPs. Substrates of S10, S20 and S30 are according to the diameters of 10, 20 and 30 nm, respectively.
Figure 2 shows the Raman spectra acquired from $1\times 10^{-4}$ M MB on AuNP/BDD substrates. The spectrum of MB powder placed on a glass slide is shown for comparison. The peaks at 447, 480, 500, 595, 1394, 1433, and 1624 cm$^{-1}$ appearing in the figure correspond to the main MB characteristic peaks.[16,17] Notably, the characteristic peaks of the diamond are absent because of the strong SERS signals of the probe molecule inhibiting the diamond peaks. Obviously, all the vibrational modes of the MB molecule on AuNP/BDD substrates are significantly enhanced in comparison with those of MB powder on glass, indicating the existence of the SERS phenomenon.[12,16,17] Enhancement factor (EF) is evaluated by[11,12,18] $$ {\rm EF}= \frac{I_{\rm SERS}\times N_{\rm NR}}{I_{\rm NR}\times N_{\rm SERS}}, $$ where $I_{\rm SERS}$ is the intensity of the selected Raman peaks in the SERS spectrum of MB molecules, $I_{\rm NR}$ is the intensity of the same mode in the normal spectra, and $N_{\rm SERS}$ and $N_{\rm NR}$ are the average numbers of the molecules illuminated by the focused laser spot under SERS and normal Raman conditions, respectively. Here the band at 447 cm$^{-1}$ was selected to estimate the EF.[19,20] The EF values of the substrates S10, S20, and S30 are calculated to be $3.38\times 10^{5}$, $9.0\times 10^{5}$, and $5.19\times 10^{4}$, respectively, which means that the SERS performance of the substrate S20 is higher than the others. In our previous work, the semiconductor SERS substrate based on bared BDD films had an EF of $1.9\times 10^{4}$ for the MB detection.[12] Therefore, the introduction of AuNPs here significantly improves the SERS performances, especially with an optimized AuNPs (20 nm in diameter). It is known that electromagnetic enhancement (EM) and chemical enhancement (CE) are the main mechanisms to explain the SERS phenomenon.[5,11,12,21–23] The EM mechanism involves the enhanced optical field and localized surface plasmon resonance of the substrate.[5] The CE mechanism (or charge transfer (CT)) is related to the formation and transfer process of charges between the substrate and chemisorbed molecules.[7,24,25] For the AuNP/BDD substrates, it is speculated that the two mechanisms are attributed to the SERS enhancement. The CT process depends on vibronic coupling of the conduction band/valence band of diamond and the excited/ground state of a molecule probe.[26] For diamond, the location of conduction-band minimum (CBM) and valence-band maximum (VBM) are related to the state of surface termination. Here the BDD is oxygen-terminated resulting from the annealing process, and its CBM and VBM are localized at $-$0.5 and $-$6.0 eV, corresponding to the vacuum level of zero.[26] For MB molecules, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are $-$5.4 and $-$3.6 eV, respectively. Thus, the CT processes from the BDD valence band to the LUMO of MB are allowed, with resonant excitation energy of $\sim $2.0–2.4 eV. As a result, SERS occurs under 633 nm (1.95 eV) excitation, which is close to the charge-transfer transition energies. Based on the three-dimensional finite-difference time-domain (3D-FDTD) simulations, the electromagnetic fields from AuNPs are simulated to estimate the electromagnetic enhancement effect.[11,27] For the simulation, the AuNP (dielectric constant, $\varepsilon =1$) on diamond ($\varepsilon =5.76$) with hemispherical shape as observed in Fig. 1 was constructed (Fig. 3(a)).[28] The AuNP hemispheres were tightly bound to BDD, and three cases of AuNPs with diameters of 10 nm, 20 nm, and 30 nm were set. At the same time, a 633-nm plane wave source was used to propagate from the upper direction of the samples to perform photoexcitation. The localized electric field intensity ($|{\boldsymbol E}|^{2}$) distributions are shown in Figs. 3(b)–3(d) for the cases of AuNPs with various diameters. There are hot-rings along with the circular contact between the AuNP and the diamond film. The intensity $|{\boldsymbol E}|^{2}$ on the contact ring is largest for the case of hemispherical AuNP with size of 20 nm. Since the EFs of AuNP are roughly proportional to $|{\boldsymbol E}|^{4}$,[29] the Raman enhancement is thus dependent on size of AuNPs on diamond. Taking into account synergistic enhancement effects of BDD and AuNPs, the AuNP/BDD substrates with varying AuNPs show different EFs, as observed in Fig. 2.
cpl-37-6-068102-fig3.png
Fig. 3. Schematic diagram of an AuNP on diamond film (a), (b)–(d) are the FDTD simulations of electric field intensity distribution for AuNPs with varying diameters ($D=10$, 20, 30 nm) on diamond.
cpl-37-6-068102-fig4.png
Fig. 4. SERS spectra of AuNP/BDD substrates. (a) Detection limit test at varying MB probe concentrations from 10$^{-3}$ to 10$^{-8}$ M, (b) reproducibility and uniformity test from fifteen random region, (c) stability test from comparison between the as-fabricated fresh substrate and the one put in air for 120 days, (d) recyclability test for four detecting/cleaning cycles.
The examinations of detection limit, reproducibility, stability, and recyclability of SERS substrate were performed on the S20 AuNP/BDD sample having the strongest SERS. In Fig. 4(a), the SERS spectra of the MB probe show the variation in intensity of the characteristic peaks as a function of concentrations. With decreasing the MB concentration, the signals decrease monotonously, and become absent when the concentration is reduced to lower than $1.0\times 10^{-8}$ M (not shown). Therefore, a detection limit as low as 10$^{-8}$ M is achieved for the AuNP/BDD substrate. Figure 4(b) shows the Raman spectral data measured from fifteen random regions on one S20 substrate (with MB concentration of 10$^{-4}$ M). It is found that the substrate exhibits excellent SERS reproducibility and uniformity. The relative standard deviations (RSD) of the main MB peaks at 447, 1394, and 1624 cm$^{-1}$ are calculated to be 0.183, 0.187, and 0.184, respectively, revealing a high reproducibility of AuNP/BDD substrates. To investigate the stability of the AuNP/BDD substrates, one S20 substrate put in the atmospheric environments for 120 days was checked. In Fig. 4(c), the SERS spectrum is nearly the same in comparison with that from the as-fabricated fresh substrate. It is confirmed that the AuNP/BDD substrates are indeed stable even for months, related to the physical rigidity and chemical inertness of both AuNP and diamond. The recyclability of the AuNP/BDD substrate was tested in the multiple cycles of detecting/cleaning processes. After the first SERS measurement for AuNP/BDD substrate (S20), the substrate was ultrasonically cleaned for 5 min in deionized water to remove the adsorbed MB probe. Then the MB probe with the same concentration (10$^{-4}$ M) was spread on the re-cleaned surface for the second SERS measurement. Figure 4(d) shows the Raman spectra obtained for four cycles. After ultrasonic cleaning process, the characteristic peaks of MB disappeared, meaning that the MB molecules were safely moved and a clean substrate surface was presented. For each cycle, the SERS spectrum was well reproduced, revealing the reusability of AuNP/BDD substrates. In summary, AuNP/BDD SERS substrates have been prepared by annealing gold films on BDD films. A high EF of $9.0\times 10^{5}$ is achieved after optimizing the feature of deposited AuNPs, and the enhancement mechanism is related to the synergistic effects of electromagnetic enhancement and chemical enhancement. A series of spectral tests demonstrate that the AuNP/BDD SERS substrates have high sensitivity, reproducibility, stability, and recyclability. These characteristics make AuNP/BDD substrates suitable for high performance SERS detection. The results in this work would open up new opportunities for diamond in real-life applications. 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