Shaped femtosecond laser induced photoreduction for highly controllable Au nanoparticles based on localized field enhancement and their SERS applications

Chen Li
  • Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China
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, Jie Hu
  • Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China
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, Lan JiangORCID iD: https://orcid.org/0000-0003-0488-1987, Chenyang Xu
  • Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China
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, Xiaowei Li
  • Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China
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, Yunfeng Gao
  • Han’s Laser Technology Centre, Shennan Ave No. 9988, Nanshan District, Shenzhen City, Guangdong Province 518057, P.R. China
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and Liangti Qu
  • Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P.R. China
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Abstract

Gold nanoparticles (Au NPs) have a wide range of applications because of their localized surface plasmon resonance properties. Femtosecond laser is considered to be an effective method for preparing Au NPs because of its characteristics of ultrashort irradiation periods and ultrahigh intensities. In this study, a novel method is proposed to produce an Au NP-attached substrate using the spatially and temporally shaped femtosecond laser. Laser-induced periodic surface structures (LIPSS) are designed to obtain the localized optical field enhancement, which leads to the femtosecond laser spatially reshaping, enabling the deposition of Au NPs by photoreduction on silicon substrates. The Au NPs prepared by this method exhibit morphological controllability and chemical stability, especially excellent spatial selectivity and uniformity, resulting in the tunable and stable surface-enhanced Raman scattering (SERS) applications. Also, the temporally shaped femtosecond pulses are introduced to further increase the enhancement factors of the SERS. This method successfully achieves the controllable morphology synthesis and selective deposition of Au NPs on the substrate simultaneously, which provides a promising candidate for SERS substrates fabrication, and holds potential applications in optoelectronics, such as molecular detection and biosensors.

1 Introduction

Noble metal (e.g. gold (Au), platinum, and silver) nanoparticles (NPs) exhibit more favorable plasmon-dominated optical properties than conventional bulk metal because of the resonance of conduction electrons with the incident light [1], [2], [3]. This phenomenon is termed as localized surface plasmon resonance [4], [5]. Among the various noble metal NPs, Au NPs have a wide range of applications such as catalysis, molecular detection, and medicine because of their excellent chemical stability [6], [7], [8].

So far, much effort has been devoted to produce diverse Au NPs, such as chemical reduction, electrochemical deposition, and film thermal treatment [9], [10], [11]. Besides, because of the unique interaction mechanism between femtosecond laser and materials (different from traditional laser or sun light), femtosecond laser created a lot of exciting new possibilities in micro/nano structure fabrication [12], [13], [14], [15], [16]. In general, two primary methods (top-down and bottom-up) are currently available for fabricating Au NPs by femtosecond laser. Among these two methods, the top-down approach involves that the pulse laser irradiates the Au target (including bulk, powders, and precursors) through solution. Au NPs are sputtered and then dispersed in the solution [17], [18], [19]. In this technique, the laser beam propagates through the solution at first and then irradiates the interface between target and liquid, which occurs complex physical and chemical processes at the same time [20], [21]. The size of the product can be controlled by changing the laser parameters and the solutions. Through this method, small and pure Au NPs are generally yielded, but the morphology controllability of the prepared Au NPs remains limited [22], [23].

In addition, the bottom-up approach can also be utilized to prepare Au NPs. It is a photoreduction process initiated by laser and the solution containing Au3+ ions and other additives, which always occurs in the region of the laser focus. In the process, the focused laser irradiates the solution containing Au3+ ions to form the Au seeds [24], [25]. The seeds are dispersed in the solution and further grown to the Au nanocluster, whose morphology can be controlled under the influence of the surfactants and the surrounding environment [26], [27]. However, some surfactants are difficult to be degraded and are harmful to the environment. Therefore, this method always requires a complicated purification procedure before the specific application [28].

These two main-stream methods both can achieve size-controllable Au NP preparation. However, both of the aforementioned methods meet a common challenge that the Au NPs are always dispersed in the solution. In certain applications, such as surface-enhanced Raman scattering (SERS) detection, substrates are required to carry the prepared nanostructures [29]. Hamad et al. prepared NP-embedded SERS structures by femtosecond laser ablation of Si in acetone solution and then drop casted or plated the metal NPs on them, which has wide application in a variety of molecule test [30]. Ran et al. prepared silver NPs on the Si substrates by femtosecond laser-induced plasma, and it was found to have high sensitivity [31]. Powell et al. used femtosecond laser direct writing and physical deposition to prepare Si-Au particle composite structures on Si surface, which have been widely used in biological detection [32]. They contributed to the preparation of SERS substrate by using femtosecond laser to fabricate a micro-nano composite structure of metal and substrate. But the uniformity, stability, and controllability of SERS substrate are still needed to be improved. Therefore, a cost-effective, efficient method of the Au NP-attached SERS substrate fabrication containing a controllable position and morphology remains challenging.

In this study, a novel method was proposed to produce an Au NP-attached substrate using a spatially and temporally shaped femtosecond laser. Laser induced periodic surface structures (LIPSS) were designed to obtain the localized optical field enhancement (LOFE), which led to femtosecond laser reshaping, enabling the deposition of Au NPs on silicon substrates with excellent spatial selectivity and uniformity. Also, the finite difference time domain (FDTD) was used to simulate optical fields, which demonstrated the existence of LOFE. In our method, the morphology of the Au particles can be easily controlled by adjusting the laser parameters, which was classified into cluster-like, transitional-state, and sphere-like structures. Au NP-attached substrates with various particle morphologies and spatial distributions were used for SERS detection, which performed sensitive and tunable activity under the synergistic effect of physical enhancement and chemical enhancement: The enhancement factors (EFs) can be further improved to 4.3×107 by introducing temporally shaped femtosecond laser, and the Au NP-attached substrates exhibit excellent chemical stability with the intensity deviation of 5.3% in the atmosphere for 2 months. This new method facilitates the efficient, low-cost, and mask-free manufacturing of SERS substrates. Also, this new method provides a new strategy to fabricate Au NP-attached SERS substrate and holds great potential application in molecular detection and biosensors.

2 Experimental section

2.1 Preparation of precursor solution and substrates

At room temperature, 0.05 m HAuCl4 and 0.05 m KNaC4H4O6 aqueous solutions were configured. The precursor solution for Au NPs fabrication was prepared by mixing stock solutions of [AuCl4] and KNaC4H4O6 in equal parts. The sample substrate was silicon (crystal orientation: 100). Before the experiment, the samples were ultrasonically and sequentially bathed with ethanol and deionized water for 10 min each.

2.2 Fabrication process

Figure 1 shows the schematic of this process. The detailed experimental setup is provided in Figure S1. In the atmosphere, a uniform ripple structure was obtained by a femtosecond laser (Figure 1A). In the mixed solution of [AuCl4] and KNaC4H4O6, a secondary scan was performed on the primary uniform periodic section to produce Au particles through photoreduction (Figure 1B). Furthermore, the morphology of particles can be varied by adjusting laser parameters. Therefore, Au particles were attached to the periodic area from the previous treatment, whereas the untreated area did not contain any deposition (Figure 1C).

Figure 1:
Figure 1:

Schematic process of the Au NP-attached substrate fabrication.

(A) A uniform ripple structure was obtained by femtosecond laser in the air environment. (B) Femtosecond laser induced photoreduction process in the solution. (C) Shape-controlled Au NPs decorated on the treated Si substrate.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

A linear polarized femtosecond laser pulse from Ti:sapphire laser system with a duration of 35 fs, a repetition rate of 1000 Hz, and a center wavelength of 800 nm was adopted for the fabrication process. A laser pulse was divided into two pulses by a Michelson interferometer and focused at the interface between the solution and the substrate by a 10× Olympus microscope objective lens (NA=0.3). The diameter of the laser spot size at focus is approximately 6.12 μm, and the Rayleigh length of the objective in our experiment is approximately 36.67 μm. The laser fluence and interval were 0.5 J/cm2 and 2 μm, respectively. The samples were translated using a six-axis translation stage. The positioning accuracy is 1 μm in the X and Y directions. The positioning accuracy in the Z direction is 0.5 μm. The Au NP-attached substrates were rinsed with distilled water after the experiment.

2.3 Characterization

The morphology of the prepared substrate was characterized through SEM (Hitachi S-4800, Ibraraki, Japan). Three-dimensional morphology was obtained using an AFM (Dimension Edge PSS, Bruker, Inc., Karlsruhe, Germany). SERS measurements were performed by Micro-Raman spectroscopy (inVia reflex, Renishaw, Gloucestershire, London, UK). XPS was performed using a PHI Quantera X-ray photoelectron spectrometer (Physical Electronics GmbH, Ismaning, Germany).

3 Results and discussion

3.1 Selective deposition of Au NPs

Figure 2A presents the schematic of fabricating the periodic structures on Si substrate in air, and the arrow indicates the direction of the laser movement. The formation mechanism of periodic surface structures is as follows: One of the main properties of femtosecond lasers is the ultra-short time scale. During the process of femtosecond laser irradiation onto the silicon, the surface becomes metallic instantaneously when the number of excited electrons increases to a critical amount, thus producing surface plasmons (SPs). The surface plasmon polaritons (SPPs) will be generated by a superposition of the incident laser and the SPs. Finally, with the initial laser-SPPs interference and the subsequent grating-assisted SPPs-laser coupling effect [33], near-subwavelength ripples were formed on the silicon surface [34]. Figure 2B is a scanning electron microscopy (SEM) image of near-subwavelength ripples generated by the femtosecond laser. The SEM image indicates that the corrugated structure has a period of approximately 650 nm. The structure also provides a uniform carrier for the adhesion of Au NPs. Also, the surface with the corrugated Si can absorb the incident light more effectively than the planar Si, which has a LOFE effect of the subsequent laser. Therefore, we designed the corrugated Si to reshape the femtosecond laser optical field, which enables the tunable deposition of Au NPs on silicon substrates. Figure 2C depicts the schematic of the production of Au atoms through photoreduction in the solution, and the arrow indicates the direction of laser movement during the process. Figure 2D is a schematic of the reduction of Au3+ through LOFE. In conventional photoreducing process, a relatively high pulse energy is required to obtain Au NPs without reductant, which may cause irreversible damage and eruption debris on the substrate [35]. In this study, the laser energy required for photoreduction on the substrate was reduced because the corrugated structure processed in the first step can achieve LOFE, thus enabling the reduction in the corrugated structure with low pulse energy without destroying the substrate. Hydrogen radicals were generated due to the multiphoton ionization of water, causing Au3+ to be gradually reduced to Au atoms. In this way, selective deposition of Au NPs on the LOFE area was achieved. Therefore, the proposed design of surface periodic structures is imperative to obtain the spatial selectivity deposition of Au NPs.

Figure 2:
Figure 2:

Schematic and the results of Au3+ reduction in two steps.

(A) Femtosecond laser irradiation in air. (B) SEM image of periodic surface structures, with a scale bar of 5 μm. (C) Femtosecond laser irradiation in a solution containing Au3+. (D) Reduction of Au3+ because of LOFE. (E) SEM image of the square array, with a scale bar of 100 μm. (F) SEM image of the “BIT” shape, with a scale bar of 500 μm.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

To evaluate the effects of this processing method, three sets of comparative experiments were performed (more details are provided in the Supplementary Material and Figure S2). The results indicated that the selective deposition of Au particles cannot be achieved by only using a reducing agent (Figure S2A) or directly writing on the solid-liquid interface (Figure S2B). On the basis of the comparative experiment, a two-step process, as proposed in this study, is advantageous for selectively depositing uniform Au particles. It is worth noting that the flexible pattern-deposition of Au particles can be realized because of the direct writing characteristics of the femtosecond laser [36]. Figure 2E and F show the SEM images of the square array and “BIT” shape fabricated using this new method, respectively.

In order to obtain the morphology of the corrugated structure more comprehensively, atomic force microscopy (AFM) characterization was carried out. Figure 3A and B present the AFM image and cross-sectional view of the corrugated structure, which indicate that the corrugated structure has an approximately 650 nm period and 150 nm depth. The results also confirm the aforementioned SEM results (Figure 2B). To verify the LOFE property of the uniform periodic surface structure processed in the first step, the localized optical field was simulated based on FDTD method using a commercial software program (FDTD solutions). In the simulation process, we used the characterization results (period and depth) from the AFM image and simplified the femtosecond laser and ripples with the plane wave and the periodic rectangular structure, respectively (the schematic diagram of effective focal spot size position above the LIPSS surface is provided in the Supplementary Material and Figure S3). Figure 3C and D present two views of the field enhancement of the uniform periodic surface structure. Under femtosecond laser excitation, a reinforcing region can be observed, and the optical field enhancement area is present in the region that is approximately 0.4 μm above the trough of the periodic surface structure (Figure 3D). FDTD demonstrates the existence of LOFE, which is considered to cause the photoreduction under low pulse energy.

Figure 3:
Figure 3:

FDTD simulations of a local electric field induced by a plane wave (λ=800 nm) that is vertically incident on periodic surface structures.

(A) AFM image of periodic surface structures, with a scale bar of 5 μm. (B) Sectional view of the AFM image, with a scale bar of 2 μm. (C) XY section of the periodic surface structures. (D) XZ section of the periodic surface structures.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

In this study, we not only analyzed the enhancement effect of light field but also fully studied the photoreduction process of the Au NPs. The reduction process of Au ions can be attributed to the direct multiphoton interaction with a laser field-induced radical-mediated reaction. The reaction process of specific chemical reductions can be explained as follows: Free electrons and H· were produced by the multiphoton absorption of water, which induced a reduction of the Au3+ ions. Multiphoton absorption of water leads to optical breakdown, and then free electrons and H· are released (Reaction 1) [37]. The reaction of reducing radicals and hydrated electrons with ionic Au is generally the mechanism that initially reduces Au3+. Moreover, hydrogen atoms can reduce the complexity of Au3+ [38], [39]. The disproportionation reaction of Au2+ is essential to the reaction process, and Au atoms are finally obtained through stepwise reduction (Reaction 2) [40]. During this process, numerous Au ions with different valences coexist in the solution. When Au0 finally reaches supersaturation, the Au atoms coalesce with each other, self-assemble, and aggregate to form Au seeds (Reaction 3). In the reduction process, the hydroxyl group in sodium potassium tartrate can also assist in the reduction of Au ions in the solution [41]. During this process, KNaC4H4O6 acts as a kind of auxiliary reducing agent, and more details are shown in the Supplementary Material and Reaction S1.

Multiphoton absorption of water

H2OmhvH2O++eH2O++H2OH3O++OH·H3O++eH2O+H

Reduction of Au3+

Au3++HAu2++H+Au3++eAu2+Au2++Au2+Au3++Au+Au++eAu0

Atomic Au coalescence

nAu0Aun

3.2 Morphological changes of Au NPs

The morphology of Au NPs has a significant impact on their photoabsorption and physical/chemical properties. Therefore, the synthesis of Au NPs with morphological controllability is essential. As shown in Figure 4A–E, the specific morphologies of the Au particles deposited on the silicon substrate surface can be observed through SEM (see the details about energy dispersive X-ray spectroscopy (EDX) mapping results of sediments in the Supplementary Material and Figure S4) [42]. In photoreduction processing, we achieve many kinds of Au NPs morphologies through controlling the scanning speeds, which are classified into three types: Type I: At a scanning speed of 160 μm/s, cluster-like Au structures composed of slice were generated. The diameter of a single cluster is 800 nm to 1 μm, and the spacing between clusters is close to the diameter of the cluster. Type II: With the decrease in scanning speed, the edge of cluster-like structures exhibited a passivation tendency, and the clusters were gradually dispersed. We believe that Type II is a transitional state between Type I and Type III. In Type II, the size of a single cluster slightly reduces with an increase in scanning speed. Type III: When the scanning speed is approximately 20 μm/s, the cluster-like Au structures were mutated to the sphere-like Au NPs, and the sizes of Au particles were also sharply reduced. Spherical Au NPs have a relatively uniform diameter distribution of approximately 50 nm, and the particles are closely arranged (the histogram of Au NPs diameters distribution is shown in Figure S5). Therefore, by adjusting the scanning speed during the photoreduction process, morphologies of Au particles can be controlled flexibly, which paves the way for the tunable SERS application.

The detailed surface chemical properties of Type I and Type III were further studied using X-ray photoelectron spectroscopy (XPS). The Au 4f spectra for Type I (Figure 4F) and Type III (Figure 4G) regions exhibit two peaks at 84 and 87.6 eV, which confirms the existence of Au0 rather than other valence in the test area. That means the structures are formed by the native Au instead of HAuCl4 crystal. Figure 4F–G present magnified views of the typical morphology of Type I and Type III structures, respectively. In addition, we also studied the deposition rate of Au particles by adjusting the scanning speed (see more details in the Supplementary Material and Figure S6).

Figure 4:
Figure 4:

SEM images of Au structures formed at different scanning speeds.

(A) Type I: cluster-like structure. (B–D) Type II: cluster-like to sphere-like transitional state. (E) Type III: sphere-like Au NPs. The scale bars are 500 nm. Au 4f spectral regions on (F) Type I and (G) Type III structures were further studied through XPS. The scale bars of the illustrations are 250 nm.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

The mechanism of size reduction with a decrease in scanning speed could be analyzed in this experiment. Photothermal ablation, near-field enhancement, and Coulomb explosion are the three probable mechanisms [43], [44], [45], [46]. Photothermal ablation may occur when low irradiation fluence and high repetition rate are adopted, because high repetition rate may cause the heat accumulation, which is characterized by the melting and evaporation of the surface of particles [44], [47]. Near-field enhancement mechanism frequently requires high laser fluence and produces intermediates [43], [45]. In Coulomb explosion, numerous electrons are ejected to generate multiple ionized NPs that undergo spontaneous fission because of charge repulsion. The Coulomb explosion mechanism is generally considered to be a dominant factor in femtosecond fragmentation [48], [49].

In this study, when the femtosecond laser fluence was constant, with a decrease in scanning speed, the morphology and size of the Au structures varied dramatically. The Coulomb explosion is considered as the main mechanism of the size mutation in this research. The detailed reasons are as follows: (1) The size of Au particles was drastically reduced. Scanning speed reduction indicates that the material surface undergoes more laser irradiation doses per unit area. If there is no phase change causing fragmentation, as the soaking time increases, the size of Au clusters would increase rather than reduce sharply. (2) The laser fluence was 0.5 J/cm2, which is less than the energy required to generate a near-field enhancement. (3) The sphere-like Au NPs are relatively uniform in size. Compared with photothermal ablation and near-field enhancement, the particles produced by the Coulomb explosion do not exist intermediates any more. This change is generally considered as indirect evidence of the Coulomb explosion mechanism.

According to the above mechanism analysis, the whole process can be summarized in Figure 5. Figure 5A–D show the primary growth process of cluster-like Au structures at a scanning speed of 160 μm/s. First, under the combined action of the femtosecond laser and reducing agent, Au ions dispersed in the mixed solution are reduced and formed Au seeds gradually (Figure 5A). Subsequently, the Au seeds further adsorb the surrounding Au atoms in the solution and form large aggregations (Figure 5B). Aggregations grow under the influence of the surrounding environment, and the contact between seed particles at the accurate crystallographic alignment then results in oriented attachment forming plate-like structures (Figure 5C) [50]. Finally, the aggregations of small plates with additional seeds form cluster-like Au particles upon the continuous action of free radicals in the solution (Figure 5D). The constant femtosecond laser energy with the decreasing scanning speed indicates an increase in the laser power density. The Au seeds are then completely grown into cluster-like structures at the same position, and the edge is passivated by subsequent pulses. With the further reduction in the scanning speed, the Coulomb explosion occurs when the scanning speed is approximately 20 μm/s. Figure 5E–H show the schematic of the Coulomb explosion process. Taking one of the Au nanosheets on the cluster-like structures as an example, an Au nanosheet absorbs photon energy during the continuous injection of the laser pulse at a low scanning speed (Figure 5E). Subsequently, a large amount of electrons are rapidly injected and multiple charged NPs are formed (Figure 5F). The charged NPs undergo fission because of charge repulsion (Figure 5G). Finally, small-sized sphere-like Au NPs are formed (Figure 5H).

Figure 5:
Figure 5:

Schematic of the morphological transformation of Au particles.

(A) Au seeds are formed during irradiation. (B) Au seeds adsorb the surrounding Au atoms to form larger aggregates. (C) Contact between seed particles at the accurate crystallographic alignment results in oriented attachment, thus creating plate-like structures. (D) Au cluster-like structures were formed. (E–H) Schematic of the Coulomb explosion.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

3.3 Highly sensitive and tunable SERS applications for Au NPs

Au NPs are widely used in various fields. Among them, controllable Au NPs are always used for molecular detection [51]. In this study, we took the application of Au NPs in SERS as an example to study the properties of this highly controllable structure. The obtained shape-controlled Au NP-attached substrates were employed as SERS substrates. As can be seen from the full spectrum of XPS in the processing area (Figure S7), the result shows that the surface of the SERS substrate manufactured through this method was clean and the solution ions were easily removed. R6G aqueous solution was dropped onto the substrate, which was used as a detection molecule. The Raman spectra of R6G exhibited sharp peaks at 611, 776, 1180, 1360, 1509, and 1650 cm−1. Three types of Au-silicon structures produced through the two-step process were dripping with R6G aqueous solution with a concentration of 1×10−6m for Raman signal detection. For comparison, a smooth, bare silicon wafer that was used as a reference sample was dropped with R6G aqueous solution with a concentration of 1×10−2m. According to the SERS spectra (Figure 6A), the Raman enhancement order for the structures with three morphologies is Type III>Type II>Type I. The signal for R6G was not apparent in the reference sample.

Figure 6:
Figure 6:

Raman spectra of R6G obtained on three types of substrates.

(A–B) Comparison of the peak values of three different types. All scale bars are 200 nm.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

EFs were calculated using the Raman peak at 1360 cm−1 to quantitatively investigate the EFs on different substrates. The calculation formula is shown in Equation 4. ISERS and Iref indicate Raman intensities of Au NP-attached SERS substrates and reference samples, respectively. NSERS and Nref represent the number of molecules detected on Au NP-attached SERS substrates and reference samples, respectively [11], [52].

EF=ISERSIrefNrefNSERS

The calculated EFs for Types I, II, and III structures were 2.5×106, 2.1×107, 3.5×107, respectively (the SERS results of different scanning speeds (lower than 20 μm/s) are provided in the Supplementary Material and Figure S8). For single molecule detection, the EFs of the substrates are always required to reach 106 [53]. Therefore, EFs of the controllable Au NP-attached substrates prepared in this new method are sufficiently high for molecular level measurements. The measured results demonstrate highly sensitive and tunable SERS applications [54].

The peaks of the three types were measured at 1360 cm−1 (Figure 6B) to study the relationship between morphologies and EFs. The spectra intensity of Type III surfaces is 1800% and 150% higher than that of Type I and Type II surfaces, respectively. Figure 6B illustrates SEM views of the microareas of the three types of substrates. To study the relationship between Raman EFs and different morphologies, the effects of the diameter of NP and nanogap were considered to evaluate SERS performance [55]. The size and spacing of Au NPs significantly affect local surface plasmon resonance. In Type III, spherical Au NPs are uniformly dispersed and have a small particles size. Strong coupling was observed between NPs because of a tight arrangement, which is conducive to the further enhancement of factors. To verify this supposition, FDTD was used to simulate the electric field enhancement of different NP diameters (D) and spacing (Figures S9 and S10).

To further study SERS substrates fabricated through this new method, temporally shaped femtosecond pulses were introduced in the second step of laser processing. As Figure 7A shows, the Raman spectra of R6G was obtained for Type III substrates with different pulse delays at 0, 200, 1000, 2000, and 5000 fs, respectively. We found that the EFs reach maximum at 200 fs pulse delay. For further comparison, we also tested the Type I substrates (Figure S11). The relationship between the pulse delay and EFs in two typical structures was plotted (Figure 7B). In particular, taking Type III substrate as an example, with the increase of pulse delay, the EFs increase rapidly until a maximum of 4.3×107 at the pulse delay of approximately 200 fs. Then, with the increase of pulse delay, the EFs tend to decrease slowly. In order to explore the influence of femtosecond laser pulse delay on different types of Au NPs, Type I substrates were also analyzed. The EF trend of Type I substrates is similar to that of Type III. Therefore, we proved the significant improvement of the EFs by introducing temporally shaped femtosecond pulses. In addition, we also studied the limit of detection, repeatability, and reusability of the substrates. The results showed that the substrate processed by this method exhibited good sensitivity, repeatability, and reusability (please see more details in the Supplementary Material and Figure S12).

Figure 7:
Figure 7:

Raman spectra of R6G obtained for substrates.

(A) Raman spectra of R6G obtained for Type III substrates with different pulse delays. (B) EFs calculated for Type III and Type I substrates with different pulse delays. (C) Raman mapping image of the Type III substrate with a pulse delay of 200 fs. (D) Variations of Raman spectra over time on the Type III substrate with a pulse delay of 200 fs.

Citation: Nanophotonics 9, 3; 10.1515/nanoph-2019-0460

A brief analysis of the effects of double pulses is as follows: In the region illuminated by the femtosecond laser pulses, photon absorption efficiency is improved because of the synergistic effect of two pulses. This process generally occurs during the pulse delay of several hundred femtoseconds. Therefore, within pulse delay of 1000 fs, the content of Au particles obtained by photoreduction was increased because of higher photon absorption efficiency, which led to an increase in the EFs compared with the simple femtosecond laser. However, when the pulse delay was further increased (>1000 fs), the electron decay term became significant. The free electron decay excited by the first subpulse disappeared before the second subpulse was exposed. In temporally shaped femtosecond pulses, the first beam pulse energy was less than the single pulse energy, thereby reducing the photon absorption efficiency. Therefore, a decrease in the content of Au particles per unit of substrate area was observed during photoreduction, and the particles spacing became larger. Hence, the coupling between the particles was weakened, which considerably reduced the EFs. To verify this phenomenon, EDX was performed for a component analysis of the regions with different pulse delays (Table S1). According to the semiquantitative detection, with an increase in pulse delay from 0 to 5000 fs, the content of the Au element per unit of the substrate area first increased and then decreased. The relationship between the rate of Au NPs deposition and pulse delay was also analyzed (more details are provided in the Supplementary Material and Figure S13), the two results are consistent, which can indirectly validate the speculation.

The stability and repeatability of the SERS substrate is also critical. In this study, SERS repeatability was measured using a region of the integral intensity of 20×20 μm at 1360 cm−1. A total of 441 data points were acquired with a uniform spacing of 1 μm in the selected area. The mapping result demonstrates the high uniformity of the EFs on the Au NP-attached substrates. The Au NP-attached substrates exhibited superior chemical stability with the highest intensity deviation of 5.3% on exposure to air environment for 2 months (Figure 7C and D).

4 Conclusions

In conclusion, a novel method was proposed to produce an Au NP-attached substrate using a spatially and temporally shaped femtosecond laser. LIPSS were designed to obtain the LOFE, which led to femtosecond laser reshaping, enabling the deposition of Au NPs on silicon substrates with excellent spatial selectivity and uniformity. FDTD was used to simulate optical fields, which demonstrated the existence of LOFE. Besides, the morphology of the Au particles can be easily controlled by adjusting the laser parameters, which was classified into cluster-like, transitional-state, and sphere-like structures. Au NP-attached substrates with various particle morphologies and spatial distributions were used for SERS detection, which performed sensitive and tunable activity: The enhancement factors (EFs) can be further improved to 4.3×107 by introducing temporally shaped femtosecond laser, and the Au NP-attached substrates exhibit excellent chemical stability with the intensity deviation of 5.3% in the atmosphere for 2 months. This new method facilitates the efficient, low cost, and mask-free manufacturing of SERS substrates. This work explores the mechanism including formation and deformation of metal NPs in the process of photoreduction through femtosecond laser irradiation and facilitates people to deeply understand the process of the interaction between light and materials. Furthermore, this new method provides a new strategy to fabricate Au NP-attached SERS substrate and holds great potential application in molecular detection and biosensors.

Conflict of interest: The authors declare no conflict of interest.

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    Cao W, Jiang L, Hu J, Wang AD, Li XW, Lu YF. Optical field enhancement in Au nanoparticle-decorated nanorod arrays prepared by femtosecond laser and their tunable surface-enhanced Raman scattering applications. Acs Appl Mater Interface 2018;10:1297–305.

  • [12]

    Liu Q, Zhang N, Yang JJ, Qiao HZ, Guo CL. Direct fabricating large-area nanotriangle structure arrays on tungsten surface by nonlinear lithography of two femtosecond laser beams. Opt Express 2018;26:11718–27.

  • [13]

    Jiang L, Wang AD, Li B, Cui TH, Lu YF. Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application. Light-Sci Appl 2018;7:27.

  • [14]

    Avila OI, Almeida JMP, Henrique FR, et al. Femtosecond-laser direct writing for spatially localized synthesis of PPV. J Mater Chem C 2017;5:3579–84.

  • [15]

    Bhardwaj AK, Shukla A, Maurya S, et al. Direct sunlight enabled photo-biochemical synthesis of silver nanoparticles and their bactericidal efficacy: photon energy as key for size and distribution control. J Photochem Photobiol B-Biol 2018;188:42–9.

  • [16]

    Vorobyev AY, Guo CL. Laser turns silicon superwicking. Opt Express 2010;18:6455–60.

  • [17]

    Amendola V, Meneghetti M. Laser ablation synthesis in solution and size manipulation of noble metal nanoparticles. Phys Chem Chem Phys 2009;11:3805–21.

  • [18]

    Podagatlapalli GK, Hamad S, Rao SV. Trace-level detection of secondary explosives using hybrid silver-gold nanoparticles and nanostructures achieved with femtosecond laser ablation. J Phys Chem C 2015;119:16972–83.

  • [19]

    Zeng HB, Du XW, Singh SC, et al. Nanomaterials via laser ablation/irradiation in liquid: a review. Adv Funct Mater 2012;22:1333–53.

  • [20]

    Xiao J, Liu P, Wang CX, Yang GW. External field-assisted laser ablation in liquid: an efficient strategy for nanocrystal synthesis and nanostructure assembly. Prog Mater Sci 2017;87:140–220.

  • [21]

    Podagatlapalli GK, Hamad S, Sreedhar S, Tewari SP, Rao SV. Fabrication and characterization of aluminum nanostructures and nanoparticles obtained using femtosecond ablation technique. Chem Phys Lett 2012;530:93–7.

  • [22]

    Zhang KH, Zhang JB, Jiang L, et al. Ablation enhancement of metal in ultrashort double-pulse experiments. Appl Phys Lett 2018;112:5.

  • [23]

    Amendola V, Meneghetti M. What controls the composition and the structure of nanomaterials generated by laser ablation in liquid solution? Phys Chem Chem Phys 2013;15:3027–46.

  • [24]

    Odhner JH, Tibbetts KM, Tangeysh B, Wayland BB, Levis RJ. Mechanism of improved Au nanoparticle size distributions using simultaneous spatial and temporal focusing for femtosecond laser irradiation of aqueous KAuCl4. J Phys Chem C 2014;118:23986–95.

  • [25]

    Tangeysh B, Tibbetts KM, Odhner JH, Wayland BB, Levis RJ. Gold nanoparticle synthesis using spatially and temporally shaped femtosecond laser pulses: post-irradiation auto-reduction of aqueous AuCl4(−). J Phys Chem C 2013;117:18719–27.

  • [26]

    Boisselier E, Astruc D. Gold nanoparticles in nanomedicine: preparations, imaging, diagnostics, therapies and toxicity. Chem Soc Rev 2009;38:1759–82.

  • [27]

    de Matos RA, Cordeiro TD, Samad RE, Vieira NJD, Courrol LC. Green synthesis of gold nanoparticles of different sizes and shapes using agar-agar water solution and femtosecond pulse laser irradiation. Appl Phys A-Mater Sci Process 2012;109: 737–41.

  • [28]

    Herbani Y, Nakamura T, Sato S. Synthesis of near-monodispersed Au-Ag nanoalloys by high intensity laser irradiation of metal ions in hexane. J Phys Chem C 2011;115:21592–8.

  • [29]

    Ma ZC, Zhang YL, Han B, et al. Femtosecond laser direct writing of plasmonic Ag/Pd alloy nanostructures enables flexible integration of robust SERS substrates. Adv Mater Technol 2017;2:7.

  • [30]

    Hamad S, Moram SSB, Yendeti B, et al. Femtosecond laser-induced, nanoparticle-embedded periodic surface structures on crystalline silicon for reproducible and multi-utility SERS platforms. Acs Omega 2018;3:18420–32.

  • [31]

    Ran P, Jiang L, Li X, Li B, Zuo P, Lu YF. Femtosecond photon-mediated plasma enhances photosynthesis of plasmonic nanostructures and their SERS applications. Small 2019;15:1804899.

  • [32]

    Powell JA, Venkatakrishnan K, Tan B. Hybridized enhancement of the SERS detection of chemical and bio-marker molecules through Au nanosphere ornamentation of hybrid amorphous/crystalline Si nanoweb nanostructure biochip devices. J Mat Chem B 2016;4:5713–28.

  • [33]

    Huang M, Zhao FL, Cheng Y, Xu NS, Xu ZZ. Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser. ACS Nano 2009;3: 4062–70.

  • [34]

    Huang J, Jiang L, Li XW, et al. Fabrication of highly homogeneous and controllable nanogratings on silicon via chemical etching-assisted femtosecond laser modification. Nanophotonics 2019;8:869–78.

  • [35]

    Kandidov VP, Kosareva OG, Golubtsov IS, et al. Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation). Appl Phys B-Lasers Opt 2003;77:149–65.

  • [36]

    Liu LP, Yang D, Wan WP, Yang H, Gong QH, Li Y. Fast fabrication of silver helical metamaterial with single-exposure femtosecond laser photoreduction. Nanophotonics 2019;8:1087–93.

  • [37]

    Elles CG, Shkrob IA, Crowell RA, Bradforth SE. Excited state dynamics of liquid water: insight from the dissociation reaction following two-photon excitation. J Chem Phys 2007;126:8.

  • [38]

    Belloni J, Mostafavi M, Remita H, Marignier JL, Delcourt MO. Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids. New J Chem 1998;22:1239–55.

  • [39]

    Gachard E, Remita H, Khatouri J, Keita B, Nadjo L, Belloni J. Radiation-induced and chemical formation of gold clusters. New J Chem 1998;22:1257–65.

  • [40]

    Dey GR, El Omar AK, Jacob JA, Mostafavi M, Belloni J. Mechanism of trivalent gold reduction and reactivity of transient divalent and monovalent gold ions studied by gamma and pulse radiolysis. J Phys Chem A 2011;115:383–91.

  • [41]

    Thakkar KN, Mhatre SS, Parikh RY. Biological synthesis of metallic nanoparticles. Nanomed-Nanotechnol Biol Med 2010;6:257–62.

  • [42]

    Kutrovskaya S, Arakelian S, Kucherik A, Osipov A, Evlyukhin A, Kavokin AV. The synthesis of hybrid gold-silicon nano particles in a liquid. Sci Rep 2017;7:10284.

  • [43]

    Tan DZ, Zhou SF, Qiu JR, Khusro N. Preparation of functional nanomaterials with femtosecond laser ablation in solution. J Photochem Photobiol C-Photochem Rev 2013;17:50–68.

  • [44]

    Link S, Burda C, Nikoobakht B, El-Sayed MA. Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses. J Phys Chem B 2000;104: 6152–63.

  • [45]

    Plech A, Kotaidis V, Lorenc M, Boneberg J. Femtosecond laser near-field ablation from gold nanoparticles. Nat Phys 2006;2:44–7.

  • [46]

    Hashimoto S, Werner D, Uwada T. Studies on the interaction of pulsed lasers with plasmonic gold nanoparticles toward light manipulation, heat management, and nanofabrication. J Photochem Photobiol C-Photochem Rev 2012;13:28–54.

  • [47]

    Link S, Burda C, Mohamed MB, Nikoobakht B, El-Sayed MA. Laser photothermal melting and fragmentation of gold nanorods: energy and laser pulse-width dependence. J Phys Chem A 1999;103:1165–70.

  • [48]

    Werner D, Hashimoto S. Improved working model for interpreting the excitation wavelength- and fluence-dependent response in pulsed laser-induced size reduction of aqueous gold nanoparticles. J Phys Chem C 2011;115:5063–72.

  • [49]

    Werner D, Furube A, Okamoto T, Hashimoto S. Femtosecond laser-induced size reduction of aqueous gold nanoparticles: in situ and pump-probe spectroscopy investigations revealing Coulomb explosion. J Phys Chem C 2011;115:8503–12.

  • [50]

    Tangeysh B, Tibbetts KM, Odhner JH, Wayland BB, Levis RJ. Triangular gold nanoplate growth by oriented attachment of Au seeds generated by strong field laser reduction. Nano Lett 2015;15:3377–82.

  • [51]

    Shiota M, Naya M, Yamamoto T, et al. Gold-nanofeve surface-enhanced Raman spectroscopy visualizes hypotaurine as a robust anti-oxidant consumed in cancer survival. Nat Commun 2018;9:16.

  • [52]

    He CY, Bai H, Yi WC, et al. A highly sensitive and stable SERS substrate using hybrid tungsten dioxide/carbon ultrathin nanowire beams. J Mater Chem C 2018;6:3200–5.

  • [53]

    Jang YH, Chung K, Quan LN, et al. Configuration-controlled Au nanocluster arrays on inverse micelle nano-patterns: versatile platforms for SERS and SPR sensors. Nanoscale 2013;5:12261–71.

  • [54]

    Wang AD, Jiang L, Li XW, et al. Low-adhesive superhydrophobic surface-enhanced Raman spectroscopy substrate fabricated by femtosecond laser ablation for ultratrace molecular detection. J Mat Chem B 2017;5:777–84.

  • [55]

    Zhao XH, Deng M, Rao GF, et al. High-performance SERS substrate based on hierarchical 3D Cu nanocrystals with efficient morphology control. Small 2018;14:1802477.

Footnotes

Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2019-0460).

If the inline PDF is not rendering correctly, you can download the PDF file here.

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    Zhu J, Du HF, Zhang Q, et al. SERS detection of glucose using graphene-oxide-wrapped gold nanobones with silver coating. J Mater Chem C 2019;7:3322–34.

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    Mohanty US. Electrodeposition: a versatile and inexpensive tool for the synthesis of nanoparticles, nanorods, nanowires, and nanoclusters of metals. J Appl Electrochem 2011;41: 257–70.

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    Cao W, Jiang L, Hu J, Wang AD, Li XW, Lu YF. Optical field enhancement in Au nanoparticle-decorated nanorod arrays prepared by femtosecond laser and their tunable surface-enhanced Raman scattering applications. Acs Appl Mater Interface 2018;10:1297–305.

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    Liu Q, Zhang N, Yang JJ, Qiao HZ, Guo CL. Direct fabricating large-area nanotriangle structure arrays on tungsten surface by nonlinear lithography of two femtosecond laser beams. Opt Express 2018;26:11718–27.

  • [13]

    Jiang L, Wang AD, Li B, Cui TH, Lu YF. Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application. Light-Sci Appl 2018;7:27.

  • [14]

    Avila OI, Almeida JMP, Henrique FR, et al. Femtosecond-laser direct writing for spatially localized synthesis of PPV. J Mater Chem C 2017;5:3579–84.

  • [15]

    Bhardwaj AK, Shukla A, Maurya S, et al. Direct sunlight enabled photo-biochemical synthesis of silver nanoparticles and their bactericidal efficacy: photon energy as key for size and distribution control. J Photochem Photobiol B-Biol 2018;188:42–9.

  • [16]

    Vorobyev AY, Guo CL. Laser turns silicon superwicking. Opt Express 2010;18:6455–60.

  • [17]

    Amendola V, Meneghetti M. Laser ablation synthesis in solution and size manipulation of noble metal nanoparticles. Phys Chem Chem Phys 2009;11:3805–21.

  • [18]

    Podagatlapalli GK, Hamad S, Rao SV. Trace-level detection of secondary explosives using hybrid silver-gold nanoparticles and nanostructures achieved with femtosecond laser ablation. J Phys Chem C 2015;119:16972–83.

  • [19]

    Zeng HB, Du XW, Singh SC, et al. Nanomaterials via laser ablation/irradiation in liquid: a review. Adv Funct Mater 2012;22:1333–53.

  • [20]

    Xiao J, Liu P, Wang CX, Yang GW. External field-assisted laser ablation in liquid: an efficient strategy for nanocrystal synthesis and nanostructure assembly. Prog Mater Sci 2017;87:140–220.

  • [21]

    Podagatlapalli GK, Hamad S, Sreedhar S, Tewari SP, Rao SV. Fabrication and characterization of aluminum nanostructures and nanoparticles obtained using femtosecond ablation technique. Chem Phys Lett 2012;530:93–7.

  • [22]

    Zhang KH, Zhang JB, Jiang L, et al. Ablation enhancement of metal in ultrashort double-pulse experiments. Appl Phys Lett 2018;112:5.

  • [23]

    Amendola V, Meneghetti M. What controls the composition and the structure of nanomaterials generated by laser ablation in liquid solution? Phys Chem Chem Phys 2013;15:3027–46.

  • [24]

    Odhner JH, Tibbetts KM, Tangeysh B, Wayland BB, Levis RJ. Mechanism of improved Au nanoparticle size distributions using simultaneous spatial and temporal focusing for femtosecond laser irradiation of aqueous KAuCl4. J Phys Chem C 2014;118:23986–95.

  • [25]

    Tangeysh B, Tibbetts KM, Odhner JH, Wayland BB, Levis RJ. Gold nanoparticle synthesis using spatially and temporally shaped femtosecond laser pulses: post-irradiation auto-reduction of aqueous AuCl4(−). J Phys Chem C 2013;117:18719–27.

  • [26]

    Boisselier E, Astruc D. Gold nanoparticles in nanomedicine: preparations, imaging, diagnostics, therapies and toxicity. Chem Soc Rev 2009;38:1759–82.

  • [27]

    de Matos RA, Cordeiro TD, Samad RE, Vieira NJD, Courrol LC. Green synthesis of gold nanoparticles of different sizes and shapes using agar-agar water solution and femtosecond pulse laser irradiation. Appl Phys A-Mater Sci Process 2012;109: 737–41.

  • [28]

    Herbani Y, Nakamura T, Sato S. Synthesis of near-monodispersed Au-Ag nanoalloys by high intensity laser irradiation of metal ions in hexane. J Phys Chem C 2011;115:21592–8.

  • [29]

    Ma ZC, Zhang YL, Han B, et al. Femtosecond laser direct writing of plasmonic Ag/Pd alloy nanostructures enables flexible integration of robust SERS substrates. Adv Mater Technol 2017;2:7.

  • [30]

    Hamad S, Moram SSB, Yendeti B, et al. Femtosecond laser-induced, nanoparticle-embedded periodic surface structures on crystalline silicon for reproducible and multi-utility SERS platforms. Acs Omega 2018;3:18420–32.

  • [31]

    Ran P, Jiang L, Li X, Li B, Zuo P, Lu YF. Femtosecond photon-mediated plasma enhances photosynthesis of plasmonic nanostructures and their SERS applications. Small 2019;15:1804899.

  • [32]

    Powell JA, Venkatakrishnan K, Tan B. Hybridized enhancement of the SERS detection of chemical and bio-marker molecules through Au nanosphere ornamentation of hybrid amorphous/crystalline Si nanoweb nanostructure biochip devices. J Mat Chem B 2016;4:5713–28.

  • [33]

    Huang M, Zhao FL, Cheng Y, Xu NS, Xu ZZ. Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser. ACS Nano 2009;3: 4062–70.

  • [34]

    Huang J, Jiang L, Li XW, et al. Fabrication of highly homogeneous and controllable nanogratings on silicon via chemical etching-assisted femtosecond laser modification. Nanophotonics 2019;8:869–78.

  • [35]

    Kandidov VP, Kosareva OG, Golubtsov IS, et al. Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation). Appl Phys B-Lasers Opt 2003;77:149–65.

  • [36]

    Liu LP, Yang D, Wan WP, Yang H, Gong QH, Li Y. Fast fabrication of silver helical metamaterial with single-exposure femtosecond laser photoreduction. Nanophotonics 2019;8:1087–93.

  • [37]

    Elles CG, Shkrob IA, Crowell RA, Bradforth SE. Excited state dynamics of liquid water: insight from the dissociation reaction following two-photon excitation. J Chem Phys 2007;126:8.

  • [38]

    Belloni J, Mostafavi M, Remita H, Marignier JL, Delcourt MO. Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids. New J Chem 1998;22:1239–55.

  • [39]

    Gachard E, Remita H, Khatouri J, Keita B, Nadjo L, Belloni J. Radiation-induced and chemical formation of gold clusters. New J Chem 1998;22:1257–65.

  • [40]

    Dey GR, El Omar AK, Jacob JA, Mostafavi M, Belloni J. Mechanism of trivalent gold reduction and reactivity of transient divalent and monovalent gold ions studied by gamma and pulse radiolysis. J Phys Chem A 2011;115:383–91.

  • [41]

    Thakkar KN, Mhatre SS, Parikh RY. Biological synthesis of metallic nanoparticles. Nanomed-Nanotechnol Biol Med 2010;6:257–62.

  • [42]

    Kutrovskaya S, Arakelian S, Kucherik A, Osipov A, Evlyukhin A, Kavokin AV. The synthesis of hybrid gold-silicon nano particles in a liquid. Sci Rep 2017;7:10284.

  • [43]

    Tan DZ, Zhou SF, Qiu JR, Khusro N. Preparation of functional nanomaterials with femtosecond laser ablation in solution. J Photochem Photobiol C-Photochem Rev 2013;17:50–68.

  • [44]

    Link S, Burda C, Nikoobakht B, El-Sayed MA. Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses. J Phys Chem B 2000;104: 6152–63.

  • [45]

    Plech A, Kotaidis V, Lorenc M, Boneberg J. Femtosecond laser near-field ablation from gold nanoparticles. Nat Phys 2006;2:44–7.

  • [46]

    Hashimoto S, Werner D, Uwada T. Studies on the interaction of pulsed lasers with plasmonic gold nanoparticles toward light manipulation, heat management, and nanofabrication. J Photochem Photobiol C-Photochem Rev 2012;13:28–54.

  • [47]

    Link S, Burda C, Mohamed MB, Nikoobakht B, El-Sayed MA. Laser photothermal melting and fragmentation of gold nanorods: energy and laser pulse-width dependence. J Phys Chem A 1999;103:1165–70.

  • [48]

    Werner D, Hashimoto S. Improved working model for interpreting the excitation wavelength- and fluence-dependent response in pulsed laser-induced size reduction of aqueous gold nanoparticles. J Phys Chem C 2011;115:5063–72.

  • [49]

    Werner D, Furube A, Okamoto T, Hashimoto S. Femtosecond laser-induced size reduction of aqueous gold nanoparticles: in situ and pump-probe spectroscopy investigations revealing Coulomb explosion. J Phys Chem C 2011;115:8503–12.

  • [50]

    Tangeysh B, Tibbetts KM, Odhner JH, Wayland BB, Levis RJ. Triangular gold nanoplate growth by oriented attachment of Au seeds generated by strong field laser reduction. Nano Lett 2015;15:3377–82.

  • [51]

    Shiota M, Naya M, Yamamoto T, et al. Gold-nanofeve surface-enhanced Raman spectroscopy visualizes hypotaurine as a robust anti-oxidant consumed in cancer survival. Nat Commun 2018;9:16.

  • [52]

    He CY, Bai H, Yi WC, et al. A highly sensitive and stable SERS substrate using hybrid tungsten dioxide/carbon ultrathin nanowire beams. J Mater Chem C 2018;6:3200–5.

  • [53]

    Jang YH, Chung K, Quan LN, et al. Configuration-controlled Au nanocluster arrays on inverse micelle nano-patterns: versatile platforms for SERS and SPR sensors. Nanoscale 2013;5:12261–71.

  • [54]

    Wang AD, Jiang L, Li XW, et al. Low-adhesive superhydrophobic surface-enhanced Raman spectroscopy substrate fabricated by femtosecond laser ablation for ultratrace molecular detection. J Mat Chem B 2017;5:777–84.

  • [55]

    Zhao XH, Deng M, Rao GF, et al. High-performance SERS substrate based on hierarchical 3D Cu nanocrystals with efficient morphology control. Small 2018;14:1802477.

Journal + Issues

Nanophotonics covers recent international research results, specific developments in the field and novel applications. Nanophotonics focuses on the interaction of photons with nano-structures, such as carbon nanotubes, nano metal particles, nano crystals, semiconductor nano dots, photonic crystals, tissue and DNA. It belongs to the top journals in the field.

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  • View in gallery

    Schematic process of the Au NP-attached substrate fabrication.

    (A) A uniform ripple structure was obtained by femtosecond laser in the air environment. (B) Femtosecond laser induced photoreduction process in the solution. (C) Shape-controlled Au NPs decorated on the treated Si substrate.

  • View in gallery

    Schematic and the results of Au3+ reduction in two steps.

    (A) Femtosecond laser irradiation in air. (B) SEM image of periodic surface structures, with a scale bar of 5 μm. (C) Femtosecond laser irradiation in a solution containing Au3+. (D) Reduction of Au3+ because of LOFE. (E) SEM image of the square array, with a scale bar of 100 μm. (F) SEM image of the “BIT” shape, with a scale bar of 500 μm.

  • View in gallery

    FDTD simulations of a local electric field induced by a plane wave (λ=800 nm) that is vertically incident on periodic surface structures.

    (A) AFM image of periodic surface structures, with a scale bar of 5 μm. (B) Sectional view of the AFM image, with a scale bar of 2 μm. (C) XY section of the periodic surface structures. (D) XZ section of the periodic surface structures.

  • View in gallery

    SEM images of Au structures formed at different scanning speeds.

    (A) Type I: cluster-like structure. (B–D) Type II: cluster-like to sphere-like transitional state. (E) Type III: sphere-like Au NPs. The scale bars are 500 nm. Au 4f spectral regions on (F) Type I and (G) Type III structures were further studied through XPS. The scale bars of the illustrations are 250 nm.

  • View in gallery

    Schematic of the morphological transformation of Au particles.

    (A) Au seeds are formed during irradiation. (B) Au seeds adsorb the surrounding Au atoms to form larger aggregates. (C) Contact between seed particles at the accurate crystallographic alignment results in oriented attachment, thus creating plate-like structures. (D) Au cluster-like structures were formed. (E–H) Schematic of the Coulomb explosion.

  • View in gallery

    Raman spectra of R6G obtained on three types of substrates.

    (A–B) Comparison of the peak values of three different types. All scale bars are 200 nm.

  • View in gallery

    Raman spectra of R6G obtained for substrates.

    (A) Raman spectra of R6G obtained for Type III substrates with different pulse delays. (B) EFs calculated for Type III and Type I substrates with different pulse delays. (C) Raman mapping image of the Type III substrate with a pulse delay of 200 fs. (D) Variations of Raman spectra over time on the Type III substrate with a pulse delay of 200 fs.