The Advanced Applications of 2D Materials in SERS: Comparison
Please note this is a comparison between Version 2 by Jessie Wu and Version 1 by Yansheng Liu.

Surface-enhanced Raman scattering (SERS) as a label-free, non-contact, highly sensitive, and powerful technique has been widely applied in determining bio- and chemical molecules with fingerprint recognitions. 2-dimensional (2D) materials with layered structures, tunable optical properties, good chemical/physical stabilities, and strong charge–transfer interaction with molecules have attracted researchers’ interests. Two-D materials with a large and flat surface area, as well as good biocompatibility have been considered promising candidates in SERS and widely applied in chemical and bio-applications. It is well known that the noble metallic nanostructures with localized surface plasmon effects dominate the SERS performance. The combination of noble metallic nanostructure with 2D materials is becoming a new and attractive research domain. The SERS substrates combined with 2D materials, such as 2D graphene/metallic NPs, 2D materials@metallic core-shell structures, and metallic structure/2D materials/metallic structure are intensely studied.

  • SERS
  • 2D materials
  • metallic nanostructures

1. Graphene

Graphene has attracted lots of interest since it was mechanically exfoliated from graphite [51,52,53][1][2][3]. Graphene as a 2D material with particular electrical, optical, mechanical, and thermal properties has been widely applied in surface-enhanced Raman scattering (SERS) [19,54,55,56,57,58,59][4][5][6][7][8][9][10]. Owing to its gapless characters, graphene leads to a strong charge–transfer interaction between itself and molecules which results in the SERS enhancement. Graphene has been known as a fluorescein quencher which can efficiently reduce the fluorescein noise and improve the SERS spectra [60][11]. The π-π stacking interaction between molecules generates the chemical bond which makes graphene a molecules catcher resulting in an enrichment of molecules on the surface of graphene [51,61,62,63][1][12][13][14]. It has been reported that the EF for the single-layer graphene is 10–100 and the single-layer graphene has a higher Raman peak intensity compared to bilayer graphene [64,65][15][16]. Additionally, the graphene with a sub-nanometer thickness can act as a sub-nanometer spacer between metallic NPs which could induce extremely strong LSPR effects.
For pure graphene, it is already known that the SERS enhancement of molecules on the graphene surface within a range of 1–100 is induced by a typical CM in comparison with SiO2/Si as a reference [22][17]. To achieve the goal of high SERS performance, the hybrid metallic NPs-graphene or the metallic nanostructures-graphene system is enormously studied [66,67,68][18][19][20]. There are mainly two methods available to fabricate Au or Ag NPs on graphene. In the chemical method, the precursors such as HAuCl4 or AgNO3 are applied to form metallic NPs with varying diameters through reduction reactions [29,69][21][22]. In the physical method, the hemispherical or spherical NPs with different diameters are generated by annealing the thin metal film. Due to the easy fabrication process, graphene-metallic NPs are widely studied. For the metallic NPs-graphene SERS substrates, normally, the flat graphene surface is considered to contribute to a larger SERS enhancement. However, the situation seems not simple. In the study of Weigao Xu [7][23], the graphene was transferred on top of the half-spherical Au particles to form graphene/hemispherical Au nanoparticles SERS substrate. By applying a flat graphene film on top of metallic nanostructures, the Raman signal of copper phthalocyanine as an analyte was enlarged. On the contrary, in the study of Weigao Xu [30][24], the bent graphene surface showed stronger Raman enhancement than the flat graphene surface. In his study, the graphene was directly transferred on top of target metallic NPs, and then an annealing process was applied which was called the “active” process. By applying the annealing process, graphene was not flat anymore and stick to the spherical surface of nanoparticles tightly. Through such an “active” process which the author mentioned, the SERS performance was enhanced around 2 times. Why do the opposite situations occur? By comparison, the distance between NPs and the diameter of the NPs affected a lot. For the homogeneous and large gap nanostructured substrates, a flat graphene surface is needed to absorb more molecules around the edge of the metal structures. And for the un-uniform and small bandgap nanostructures, the flat graphene surface can let the molecules far from the “hot spots” induced by the small bandgap of adjacent nanostructures. So, the nature of surface-dependent graphene-NPs SERS performance is related to the morphology of the metallic structures. In the design and fabrication 2D materials-based SERS substrates, the gap between NPs or metallic nanostructures and their diameter should be taken into account.
Except for the NPs/graphene SERS system, the metallic nanostructures with kinds of morphologies attract lots of research interest and are intensely studied. In comparison with randomly generated metallic NPs, the nanostructures with controllable morphologies and uniform diameters can lead to more spatially stable SERS spectra which are extremely important in quantifying the number of analytes in SERS [70][25]. In the work of Zhao Xing [71][26], the graphene/gold nanorods vertical array (G/GNRs-VA) hybrid substrate was fabricated. The gold nanorod vertical array was fabricated through the seed-mediated growth method [72][27], and the graphene was transferred on top by using the wet transfer method. The G/GNRs-VA substrate showed a limit of detection (LOD) of 10−13 M and an EF of 7.9 × 108 by using R6G as the analyte. The graphene/nano-disk and graphene/nano-holes structures have been studied by Qingzhen Hao [39][28]. When the graphene was introduced, the nanostructure substrates combined with graphene showed enhancements around 3-fold or 9-fold using MB as an analyte in comparison with the bare nanohole or nanoparticle substrates, respectively. Graphene-Au nanopyramid structure was investigated by Pu Wang [27][29]. He demonstrated a graphene-Au nanopyramid heterostructure to detect dopamine and serotonin with LOD down to 10−10 M. The SERS EFs for dopamine and serotonin were 2 × 108 and 2 × 109, respectively. The graphene/Au Triangular Nanoarrays (G/TNAs) system was studied by Xingang Zhang [73][30]. By applying graphene, it efficiently improved the thermal stability of noble metal Triangular Nanoarrays. The G/TNAs substrate can be reused around 16 times without significant SERS intensity loss.
Normally the plane SERS structure generates “hot spots” in the XY plane, and its SERS performances are contributed by the gaps between horizontal patterns or the edge effects of the structures. The SERS performance of simple plane structures is usually restricted by the limited density of the hot spots [74,75][31][32]. To get high-performance substrates, the architecture of SERS substrates grows in the Z dimension is a promising method. The constructed three-dimensional (3D) structures can make use of vertical dimensions and generate LSPR in the vertical gaps could enlarge the SERS performance. For such architecture, 2D materials play an important role in generating atomic vertical bandgaps as a spacer between metallic nanostructures. The sub-nanoscale gap in the horizontal dimension between the coherent nanostructures could largely enhance the SERS performance. In the study of Xuanhua Li [76][33], graphene was applied as a nano-spacer from the metal film–metal nanoparticle coupling system. Graphene was transferred on top of 100 nm-thickness Ag film, and Ag NPs were deposited on top of graphene through a thermal evaporation process. By introducing the graphene, an atomic gap between the Ag NPs and Ag film was formed which were regarded as “hot spots”. In this structure, the Ag NPs themselves, Ag NP-Ag NP, and Ag NP-Ag film have strong electromagnetic fields. By analyzing the SERS performance of the proposed 3D structures which use the R6G as the analyte. Remarkably, the Ag NPs-graphene-Ag film system exhibited an enhancement ratio of about 1700 than normal graphene. This enhancement was nearly 115 times and 14 times larger than the Raman intensity of graphene/Ag film and graphene/Ag NPs systems, respectively. A similar strategy was applied by Hongki Kim [77][34] in which the single Au nanowire/graphene/Au film system was fabricated. In his study, graphene played a role of a sub-nanometer spacer between the nanowire and the Au film. The EF of a single nanowire on the graphene platform was calculated to be 1.18 × 106 in which Cu-phthalocyanine (CuPc) was applied as an analyte.
Except for the metal NPs/graphene/metallic film system, the metallic NPs/graphene/discontinuous metal film are investigated. In comparison with the metal film underneath, the discontinuous metal film provides more edges or sharp points that can improve the SERS enhancement performance. In the study of Yuan Zhao [46][35], the 3D hybrid system with monolayer graphene sandwiched between silver nanohole arrays and gold nanoparticles was fabricated. The nanohole structure was fabricated by using the EBL method, and the Ag was deposited on top of the holey structure to form the Ag nanoholes (Au Hs) structure. The graphene was transferred on top of the Ag nanoholes structure, and the Au NPs as deposited on top of the graphene by thermally treating a thin Au film. The proposed Au NPs/graphene/Ag NHs structure exhibited ultrahigh SERS sensitivity with a LOD down to 10−13 M. A similar structure was Ag NPs/bilayer graphene/Au nanonet structure which has been fabricated by Chonghui Li [78][36]. In his study, Ag NPs/bilayer graphene/Au nanonet was proposed to form the dense 3D hot spots. The gap in the nanonet structure was around 8.67 nm. The CVD-grown bi-layer graphene with a theoretical thickness of 0.64 nm was transferred on top of the Au nanonet structure through a wet transfer process to generate an ultrasmall gap between Au NPs and Au nanonet structures. This substrate has been applied in detecting R6G and crystal violet (CV) with LODs of 10−13 and 10−12 M respectively. In our previous paper, the porous Si3N4 films were applied as masks to form the periodic Au nano-discs (Au NDs) with varying diameters. By transferring single-layer graphene on Au NDs and fabricating Au NPs on top of graphene, the 3D Au NPs-graphene-Au NDs structures were synthesized. Through such structure, the graphene defects and non-defects induced Raman singles both were enlarged. By analyzing the two kinds of Raman intensity ratios above mentioned and applying the linear graphene defects models, we quantify the defects crystalline of CVD-grown graphene [37].
Another interesting 3D SERS system is NPs/graphene/NPs. Due to the different chemical stability of different metals, the hybrid NPs/graphene/NPs systems combining two kinds of metals were important in SERS. The Ag NPs/graphene/Au NPs system with 3D hot spots was investigated by Chao Zhang [55][6]. The Au NPs and Ag NPs were both generated by annealing the thin metal films which were deposited on either side of graphene. For this structure, the LOD for R6G and CV were 10−11 and 10−12 M respectively. Except for the pure metallic NPs, the AuAg alloy NPs also was studied. In the study of Qingyan Han [79][38], the 3D AuAg alloy NPs/graphene/AuAg alloy NPs structure was investigated in which the AuAg alloy NPs were synthesized by using mixed HAuCl4 and AgNO3 as precursors. By changing the concentration ratio of HAuCl4 and AgNO3, the absorbance of AuAg alloy NPs could be tuned. The 3D structure was fabricated layer by layer where the AuAg alloy NPs film was self-assembled in a single layer and graphene was transferred by using the wet transfer method. By optimized the ration of Au/Ag (HAuCl4/AgNO3 = 1/3), 3D AuAg alloy NPs/graphene/AuAg alloy NPs structure showed a LOD of 10−9 M using R6G as an analyte.

2. Hexagonal-Boron Nitride (h-BN)

Like graphene, a hexagonal boron nitride (h-BN) sheet is another atomic-thick 2D material. It is well known that the h-BN possesses excellent thermal stability even at high temperatures up to 800 °C in oxidative environments [80][39]. Due to the unique surface properties, h-BN can be modified by kinds of organic groups such as hydroxyl, alkoxy, amino, and amine which make h-BN a universal substrate for “clamping” metallic NPs as EM “hot spots”. Due to its unique properties, it has been reported the h-BN could prevent the oxidation of Ag NPs which efficiently increases the stabilities of Ag NPs/h-BN SERS substrate [81][40]. In the study of Na-Yeong Kim, the h-BN/Ag NPs and Au NPs/h-BN/Ag NPs were fabricated and studied [81][40]. The h-BN sheet was directly transferred onto the Ag NPs by using a dry transfer process which also was widely used in transferring exfoliated graphene. By dispersing the collied Au NPs solution, he synthesized the Au NPs/h-BN/Ag NPs sandwich structure with atomic gaps. From the SEM image of Ag NPs before and after the transfer h-BN sheet, it was clear that the monolayer h-BN was successfully transferred on top of Ag NPs. The prepared Au NPs/h-BN/Ag NPs illustrated an excellent SERS performance with an EF of 9.35 × 107 and an LOD of 10−12 M in detecting R6G. The h-BN/Ag NPs system was also studied by Dipankar Chugh with high NPs tensity [82][41]. The metal NPs were deposited on Si/SiO2 by using e-beam evaporation, and the h-BN layer was transferred on top of the NPs through a wet transfer technique. The topography image of the h-BN/Ag NPs sample was obtained by SEM and AFM measurements which illustrated a successful transfer process. In his research, the h-BN/metallic NPs showed stronger SERS performance than bare metallic NPs which revealed the superior adsorption capabilities of h-BN layers over metallic NPs. The thickness depended SERS performance of h-BN covered Au NPs system was studied by Gwangwoo Kim [83][42]. In his research, the SERS performance h-BN/Au NPs systems using CVD grown and mechanically exfoliated h-BN with different thicknesses were studied. By controlling the layer number of h-BN, the optimized thickness of h-BN for h-BN/Au NPs system was 7 nm for the exfoliated h-BN and five-layer-thickness for CVD grown h-BN. Normally, the thicker h-BN showed a potential of absorbing a larger number of molecules on its surface which cause a larger EF of SERS. However, in h-BN/Au NPs systems the thickness-dependent behavior was mainly affected by the gap between the Ag NPs which affected the EM around the surface of h-BN. When increasing the thickness of h-BN to 7 nm, the maximum of the EM of Ag NPs can be optimized at the fixed gap distance between Ag NPs.
Except for the chemistry applications, the h-BN also has been applied in biology. In the study of Jia Liu [84][43], the h-BN nanosheets synthesized by chemical and mechanical exfoliation methods were applied as a novel system in real-time monitoring CuPc labeled microRNA through the SERS technique. In his research, the CuPc as an important molecule in the photodynamic therapy process was real-time monitored by using the h-BN nanosheets as the SERS substrate. In comparison with SiO2, h-BN nanosheets exhibited higher SERS performance. Through the amplification of the SERS enhancement caused by h-BN nanosheets, the LOD of CuPc labeled miRNA-21 reached 0.7 fM in live cells. This h-BN platform illustrated a promising way of early monitoring and guiding the early therapy, realizing tumor elimination.

3. Black Phosphorus (BP)

Black phosphorus (BP) as a member of 2D materials has attractive physical properties for high-performance chemical sensing applications. Compared with graphene or MoS2, BP has superior molecular adsorption energy [85,86][44][45]. Recently, black phosphorus (BP) has attracted a lot of scientists’ interest due to BP’s unique semiconducting properties. The BP possesses extremely high hole mobility of 104 cm2 V−1 s−1 is larger than graphene [87][46], and its in-plane anisotropy with puckered orthorhombic structure provides an opportunity of designing conceptual devices and applications [88,89,90][47][48][49]. BP has a strong near-infrared (NIR) absorption and possesses a high efficiency of photothermal conversion that makes it an ideal candidate in efficient NIR photothermal cancer therapy and photoacoustic bioimaging [91,92,93][50][51][52]. BP nanosheet as a 2D material is a novel nanocarrier and photosensitizer with efficient generation of singlet oxygen for chemotherapy and photodynamic therapy [94,95,96][53][54][55]. Due to its biodegradability, intrinsic photoacoustic properties, and biocompatibility, the main application of black phosphorus is in bio-therapy, such as cancer theragnostic [97][56], chemo-photothermal therapy [91][50], photothermal therapy, photodynamic therapy, drug delivery [96][55]. Among these applications, the BP-NPs systems play an important role as a SERS substrate to reveal the bio-reaction in the cell or real-time monitor the therapy. In the research of Henan Zhao [98][57], the BP-Au NPs system has been fabricated and applied to investigate the intracellular behaviors. By reducing the HAuCl4, the Au NPs can be modified on BP sheets. The SERS experiments were carried out by applying human hepatocellular carcinoma (HepG2) cells as the model. By applying the BP-Au NPs SERS substrate, the enhanced Raman signals can be observed which gave more details inside the cell and rich fingerprint information of the intracellular biological components. Through label-free SERS images, the endocytosis mechanism of the cell using BP-Au NSs was revealed. The plasmon effect of the BP-Au NPs system not only acts as a SERS sensor in the human body but also acts as a Photothermal therapy (PTT) nano-agent applied in cancer therapy [99][58]. The BP-Au NPs system has been applied in breast cancer treatment in vitro and in vivo by Guangcun Yang and achieved the desired therapeutic outcome. The BP–Au NPs structure has a stable lamellar structure, good biocompatibility and photostability which were capable of producing sufficient hyperthermia which makes it a suitable and novel PTT nano-agents for cancer therapy. In vitro and in vivo experiments demonstrated that PTT was mediated by BP–Au nanosheets and exhibited more effective therapeutic efficacy than that based on pure BP nanosheets. During this process, the therapeutic effect in vivo was real-time monitored by the SERS technique. Through the photographs of tumor tissue and the SERS spectrum, they showed that photo-thermolysis destroyed the membrane microstructure of cancer cells and caused the intracellular redistribution of the nanocomposites [99][58].

4. SnSe


For the Raman scattering of the bio-molecules, the UV light is much easier to excite them and cause a larger polarization of biomolecules [100][59]. Besides, the UV light is much close to the electron transition spectrum which causes a strong Raman resonance and a separation of Raman spectra far from its fluorescein background region. Normally, due to the plasmon damping by inter-band transitions in the UV region only a few metals such as Rh [101[60][61],102], Ru [101][60], Rd [103][62], Co [104][63], copper [104][63], Al [105][64], and Pb showed the SERS performance in UV region.
Two-D-layered Tin Diselenide (SnSe2) nanoflakes with an indirect bandgap with a bulk bandgap of approximately 1.0 eV have strong absorption in the UV region [106,107][65][66]. The biology molecules absorbed on SnSe2 could generate large SERS enhancements under UV light excitation. For instance, the LOD of CV molecules adsorbed on SnSe2 nanoflakes could reach a concentration as low as 10−7 M [100][59]. For the SnSe2 flake, the band structure is caused by the thickness that affects the SERS performance. When the bandgap between the conduction band (CB) and valence band (VB) of the multilayer SnSe2 match the vibrational state of the molecular, the charges transfer between multilayer SnSe2 and molecules which results in the enhancements of the Raman signal.

5. MoS


Molybdenum disulfide (MoS2) as a new type of 2D-layered transition metal dichalcogenides has been widely applied in SERS. MoS2 possesses lots of unique electronic, optical and mechanical properties. The mechanisms of SERS of graphene and MoS2 are different. Graphene is a gapless semiconductor with a nonpolar C−C bond [108,109][67][68] which induces strong charge transfer. Unlike graphene, MoS2 is a semiconductor with both the top and bottom surface of the Mo atoms layer bonded by the sulfur atoms to generate a covalent Mo-S band with the polarity in the vertical direction to the surface. Due to such essential properties, both weak charge transfer and dipole−dipole coupling of MoS2 may occur and contribute to the SERS [80,110][39][69]. It has been observed that the pristine MoS2 (P-MoS2) flake does not show any SERS enhancements, and the MoS2 flake with defects showed SERS enhancement. When using the oxygen-plasma treated MoS2 (OT-MoS2) or argon-plasma treated MoS2 (AT-MoS2) nanoflakes, the SERS performance of the MoS2 could be enhanced by more than 1 magnitude. For the plasmon-treated MoS2 flake, the defects which are introduced in the T-MoS2 flake change the local surface properties of MoS2 flakes. The plasma-generated “holes” in the 2D MoS2 flake create the local dipoles which give rise to the enhanced Raman signals of molecules absorbed in such region. Further, the site of the defeat of the MoS2 flake could absorb the oxygen or H2O molecules in ambient air resulting in doped holes in the MoS2 flake. These doped sites also enhance the charge transfer process between the molecules and the MoS2 flake resulting in enhanced Raman signals [111][70]. To get high SERS performance, the categories of combing MoS2 with metallic NPs or metallic nanostructures are promising ways in enhancing the SERS performance of MoS2-based SERS substrates. The Au NPs modified exfoliated MoS2 substrates were fabricated by Shao Su [112][71]. By controlling the concentration of precursors, the Au NPs@MoS2 SERS substrates with a varying number of Au NPs were fabricated with tunable optical absorption. The optimized Au NPs@MoS2 system exhibited an EF of 8.2 × 105 and a LOD of 8.2 × 10−7 M in detecting R6G. In the study of Renu Rani [113][72], the monolayer MoS2 was cut by low-power focused laser-cutting, and the Au NPs were drop-casted on top of MoS2 forming the Au-MoS2 substrate. The edge of the MoS2 attracts major of the Au NPs due to the strong electrovalent bond between Mo and S atoms. Through the Raman mapping image, it revealed the larger SERS performance existed around the edge of the MoS2 with the aggregated Au NPs. The SERS LOD of such substrates in detecting dye molecules could reach 10−10 M, and the EF was 104.
Except for the application of MoS2 films, the MoS2 with interesting morphologies which were synthesized by using chemical methods illustrated excellent SERS performance when combining with plasmon structures. The spherical or flower-like MoS2 synthesized by using chemical methods is attracted researchers’ interest and is widely applied in SERS. MoS2 nanosphere and Au NPs-MoS2 nanosphere system were studied by Hengwei Qiu, and they illustrated extremely high SERS performance [114][73]. The MoS2 nanospheres were fabricated via using molybdenum ions sulfuration reaction proceeding. After modifying Au NPs using HAuCl4 as a percussor, the coherent Au NPs with few nanometer gaps provided a huge electromagnetic field resulting in an extremely large EF of SERS. This structure showed LODs of 10−14 M for R6G and 10−15 M for MB. Jaspal Singh used hexa-ammonium heptamolybdate tetrahydrate and thiourea as precursors to synthesize 3D MoS2 nanoflowers with the tunable surface area for the application in photocatalysis and SERS-based sensing [115][74]. By comparison with the MoS2 nanoflowers with varying surface areas, it revealed that the SERS performance showed a positive correlation with the surface area of MoS2. Besides, the pure MoS2 nanoflowers illustrated a LOD of 10−7 M in detecting RhB molecules. Another method of fabricating MoS2 nanoflower was introduced by Shib Shankar Singha [116][75]. The MoS2 nanoflower was synthesized from MoO3, and the Au NPs were then modified on the surface of MoS2 nanoflower to form Au NPs-MoS2-flowers structure. The SERS LOD of Au NPs-MoS2-flowers structure substrates could reach 10−12 M using R6G as a probe molecule. The calculated EF was around 109 in the detection of bilirubin molecules in comparison with bare MoS2 NF substrate.

6. WS


WS2 as a typical member of the TMDs family possesses strong spin-orbit coupling and band splitting deriving from the d-orbitals of transition metal [117][76]. Also, WS2 exhibited special advantages in spintronics and valleytronics [118,119][77][78]. In the study of Lan Meng [120][79], the CVD growth of WS2 film as a substrate was studied. In comparison with the SiO2/Si substrates, the Raman signal of R6G absorbed on the WS2 surface was prominently enhanced. By adding the layer number, the band structure of WS2 translates from direct to indirect bandgap. In such a case, the relaxation process changes from direct recombination to indirect recombination in which the excited electrons in a few-layer WS2 or transfer from R6G molecules would stay a longer time.
The high-performance metallic nanostructure/WS2 system is a promising candidate in SERS such as Ag@WS2 QD nanocomposites [121][80] and Au NPs/WS2 [122,123][81][82]. The 3D Au NPs/WS2@Au NPs were synthesized by Zhengyi Lu [124][83] with a LOD of 10−11 M. In this system, the bilayer WS2 was directly growing on the surface of Au NPs which generate a core-shell structure. The in-situ grown bilayer WS2 film through a thermal decomposition process was applied as a precise nano-spacer. The electrical bandgap between metallic nanostructures in the vertical direction can achieve strong plasmonic coupling which induced a tremendously enhanced local electromagnetic field resulting in extremely amplified Raman signals. The dense 3D hot spots in this hybrid plasmonic nanostructures provided more “hot spots” which were responsible for the extremely enhanced SERS performances. Besides, the tight protected Au NPs by WS2 acted as a protection film that stabilized Au NPs/WS2@Au NPs hybrid plasmonic nanostructures from oxidation and increases its reproducibility and reusability.

7. SERS Substrates with a Combination of Two Kinds of 2D Materials

7. Surface-Enhanced Raman Scattering Substrates with a Combination of Two Kinds of 2D Materials

The integration of two kinds of SERS substrates is an interesting way to get SERS substrates that illustrate higher SERS sensitivity than either one of them. The interaction between 2D materials layers plays a crucial role in enhancing SERS performance.
The Au NPs/2D MoS2/graphene system was studied by Mohammed Alamri [125][84]. The SERS performance of MoS2/graphene substrates did not show any enhancement in comparison with pure graphene substrate which was owned to weaker CM enhancement SERS performance. However, when Au NPs decorate on top of MoS2/graphene substrates forming Au NPs/MoS2/graphene system, the SERS performance was larger than Au NPs/graphene system. This result was contributed by the interlayer coupling of MoS2/graphene heterostructure. Besides, the distance between the Au atom and the S atomic consisting of layered MoS2 also affected the SERS performance of Au NPs/2D MoS2/graphene system. Through the FDTD simulation by varying the layer number of Au, it revealed that the EF could be affected by the distance between the Au atoms and S atmos. This modulation of distance at the subatomic level indeed played a critical role in further enhancing the EM effect. In the study of Samar Ali Ghopry [63][14],. the Au NPs/WS2 nano-dome/graphene van der Waals heterostructure was fabricated. In his study, the WS2 nano-domes were directly grown on the surface by thermal treating (NH4)2WS4 precursor, and Au NPs were fabricated on top of /WS2 nano-dome/graphene through a thermal treating method of thin Au film. In Au NPs/WS2 nano-dome/graphene structure, both WS2 nano-domes, and Au NPs illustrate the LSPR effect which is responsible for the Raman signal enhancement. The LOD of Au NPs/WS2 nano-dome/graphene van der Waals heterostructure can reach 1 × 10−12 M using R6G as an analyte.


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