Overcurrent Electrodeposition of Fractal Plasmonic Black Gold with Broad-Band Absorption Properties for Excitation-Immune SERS.

The dependence of plasmon resonance on the size, shape, and interparticle spacing of single, isolated nanostructures inherently limits their light-harvesting capability to a narrow spectral band. Here, we report a facile overcurrent electrodeposition strategy to prepare fractal plasmonic black gold (B-Au) with broad-band absorption properties (over 80% throughout the range of 300-1800 nm). The broad-band absorption properties are attributed to the excitation of multiple plasmons in the B-Au, which results in strong light-matter interaction over a broad-band spectral window. Consequently, the B-Au can produce strong broad-band surface-enhanced Raman scattering (SERS) regardless of the excitation light used. These findings demonstrate that the fractal B-Au allows efficient utilization of broad spectral photons and opens up exciting opportunities for highly sensitive SERS detection, photocatalysis, and photovoltaic devices.

Introduction

Plasmons are collective oscillations of free electrons in metal nanostructures in resonance with the incident light.[1] The excitation of plasmons produces a strong electric field around the nanostructures that is responsible for a series of applications including, surface-enhanced Raman scattering (SERS),[2] metal-enhanced fluorescence,[3] extraordinary optical transmission,[4] and so on. The excitation of plasmon results in strong absorption of the incident light, thereby coupling its energy into the plasmons. Upon excitation, the plasmon spontaneously decays via either a radiative pathway or a nonradiative pathway.[5] In the latter case, the decay of the plasmon transfers the accumulated energy to the electrons in the conductive band of the plasmonic material, producing highly energetic “hot electrons” (i.e., the electrons with energy exceeding thermal equilibrium-determined energy), which may be collected to produce electric current,[6] drive photocatalytic reaction,[7,8] or convert into heat,[9,10] opening up a new scheme for solar energy conversion.

However, the light-harvesting via single, isolated plasmonic nanostructures is often limited to a narrow spectral window in resonance with intrinsic plasmon frequency as dictated by particle size, shape, and interparticle spacing.[11−13] To fully utilize solar energy, a series of plasmonic black metals that can absorb light over a broad-band spectral window have been designed and fabricated. They generally include ordered nanostructures (e.g., three-dimensional porous arrays,[10] thin nanostructured metallic layer,[14] nanostructured trapezoidal arrays,[15] ultrasharp convex metal grooves,[16] and nanowire bundle arrays[17]) and nonordered nanostructures (e.g., randomly deposited nanoparticles (NPs) on a substrate,[18,19] silica capsules grafted with Au NPs,[20] and self-assembled black colloidal Au superparticles[21,22]). To date, the strong light absorption properties of plasmonic black metals have been intensively studied,[14−23] and their diverse structures offer flexible platforms for efficient plasmon-driven steam generation[10,17,23] and SERS.[22] However, it remains a great challenge to efficiently fabricate plasmonic black metals, without the need of sophisticated lithography processes or multiple assembly steps.

In the electroplating industry, it is well-known that adverse effect occurs during overcurrent deposition, which readily “burns” the deposit, producing an undesired black, rough coating, rather than the desired bright, smooth one. Here, we take advantage of this “adverse” overcurrent deposition effect to produce fractal plasmonic black gold (B-Au) with broad-band absorption properties (over 80% from 300 to 1800 nm) and further explore its SERS properties. Although previous works have theoretically studied the SERS behaviors of fractal clusters,[24,25] the aim of current work is to offer a facile strategy to fabricate plasmonic B-Au, which by chance has a fractal structure. We further demonstrate that the B-Au with fractal structure shows a strong broad-band SERS regardless of the excitation light used, which we termed as excitation-immune SERS. These results achieved thus greatly simplify the SERS detection and also may broaden the application of the B-Au in other fields, e.g., photocatalysis and photovoltaic device.

Results and Discussion

The B-Au was directly deposited on the conductive surface of Au film-coated glass slide from a commercial Au plating solution under constant overcurrent. To explore appropriate deposition conditions, we have prepared a series of Au deposits at various current densities for different durations. Figure a–e shows the scanning electron microscopy (SEM) images and corresponding photographs (insets) of the Au deposits after deposition at various current densities for 800 s. Evidently, the Au deposit shows a relatively uniform and smooth morphology at a low current density of 0.8 mA/cm2, as revealed by the bright, smooth surface of the Au thin-film electrodes. With increasing current density, the Au deposit starts to show increasingly rough morphology and eventually highly fractal nanostructures at 3.2 mA/cm2, leading to increased darkness. With further increase in the current density, the fractal nanostructure becomes more compact while their filament gradually becomes flattened, thereby displaying fading darkness. The duration of the electrodeposition process can also greatly affect the resulting structural morphology. Figure f–j displays the SEM images and the corresponding photographs (insets) for the Au deposits deposited at 3.2 mA/cm2 for different durations. With increasing electroplating time, the fractal structure gradually grows and becomes more compact, resulting in a similar color evolution from red to black and then to light black. These images vividly show the nucleation and growth process of the B-Au, revealing that the B-Au can be readily obtained by controlling the amplitude of the overcurrent and deposition duration (see Figure S1 for the resultant B-Au with different magnifications). During the electroplating process, the driving force (i.e., overpotential) for nucleation and growth is larger at overcurrent than at normal current. On one hand, this large overpotential quickly depletes the Au precursor supply near the electrode surface for growth, resulting in severe concentration polarization. On the other hand, developing a rough morphology creates a highly nonuniform field, with considerable field enhancement near sharp corners. The two factors together greatly promote the highly anisotropic deposition and development of the resultant fractal structures.[26] The fractal dimension of the B-Au can be measured using an electrochemical method,[27] which is 2.18.

Figure 1

SEM images and corresponding photographs (insets) of the Au samples deposited at (a) 0.8 mA/cm2, (b) 1.6 mA/cm2, (c) 3.2 mA/cm2, (d) 6.4 mA/cm2, (e) 12.8 mA/cm2 for 800 s, and at 3.2 mA/cm2 for (f) 300 s, (g) 500 s, (h) 800 s, (i) 1000 s, and (j) 1200 s. (k) EDS spectrum. (l) X-ray diffraction (XRD) pattern. (m) High-resolution transmission electron microscopy (HRTEM) image. (n) Selected area electron diffraction of the B-Au.

To reveal the elemental composition and crystalline phase of the B-Au, we have performed energy-dispersive X-ray spectrum (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) studies. Apart from the very minor C element that may be originated from the organic material in the plating solution, the deposit is composed of only Au element (Figure k), suggesting high purity for the B-Au. XRD pattern reveals the polycrystalline nature for the B-Au (Figure l). The pattern shows four diffraction peaks at 38.75, 44.95, 65.10, and 78.05°, which can be indexed to the (111), (200), (220), and (311) planes of polycrystalline Au, respectively. The high-resolution TEM image of the B-Au shows two dominant lattice spacings of 0.2036 and 0.1442 nm (Figure m), which can be assigned to the (200) and (220) planes of the Au, respectively. The selected area electron diffraction pattern shows four circles with different radii (Figure n), corresponding to diffractions from the (111), (200), (220), and (311) crystal planes of the B-Au, respectively. These TEM studies further confirm the polycrystalline nature for the B-Au.

With a fractal structure that contains a mixture of differently shaped and sized nanostructures with a wide distribution, the B-Au may show strong broad-band absorption capability. Figure a,b shows the absorption spectra of different Au deposits (corresponding to the samples shown in Figure a–j, respectively), along with the simulated spectrum of the B-Au for comparison. Generally, with increasing current density (plating time, 800 s) or increasing plating time (current density, 3.2 mA/cm2), the absorption of the Au deposits initially intensifies and then fades again, consistent with color change trend for different nanostructures (Figure , insets). Particularly, the B-Au shows the strongest broad-band absorption, converting a broad-band range of 300–1800 nm (exceeding 80%). Compared with other nonfractal black golds, such as plasmonic convex groove arrays (over 80%, 450–850 nm)[16] and black colloidal Au superparticles (over 97.5%, 400–2500 nm),[22] the current fractal B-Au can reach at least comparable light absorption capability in terms of absorption efficiency and band range. The strong broad-band absorption properties of the B-Au are further confirmed by the results from finite-difference time-domain (FDTD) simulations and infrared thermal thermometry. The simulated spectrum also predicts a similar broad-band absorption window covering that observed experimentally. As a result, the temperature of the B-Au quickly increases from 25 to 36.5 °C within a 2 min frame under light illumination, which, however, is not the case for flat Au film (Figure c). The infrared thermal imaging more visually shows the obvious increase in temperature for the B-Au (Figure d).

Figure 2

Absorption spectra of the Au deposits deposited (a) at different current densities for 800 s and (b) at 3.2 mA/cm2 for different times, along with the simulated spectrum of the B-Au for comparison. (c) T–t curve of the B-Au and a flat Au film under light illumination for different durations. (d) Infrared thermal image of the B-Au under light illumination and the corresponding photograph (inset) of the B-Au.

To further understand the broad-band absorption properties that the B-Au demonstrates, we have simulated the field distributions of the plasmons using the FDTD method. Figure shows the near-field distributions on the B-Au (simulated using the fractal structure shown in Figure c as a model) and the corresponding maximum field enhancement excited at different wavelengths. Notably, regardless of the excitation wavelength, the B-Au can generate abundant hot spots (i.e., the places where the field is very strong) at the edges or tips. For example, the maximum field enhancement (defined as the M = Eloc/Ein, where Eloc and Ein are the magnitudes of the localized and incident fields, respectively) is as high as 1.23 × 102 for a 785 nm excitation light. Additionally, the hot spots are widely distributed at different places, suggesting a strong light–matter interaction across the entire fractal surface. Clearly, the unique fractal nanostructure of the B-Au supports multiple plasmons that can be simultaneously excited by lights with different wavelengths, resulting in broad-band absorption properties. Relative to single, isolated plasmonic nanostructures with relatively narrow plasmons,[11−13] the B-Au with broad-band absorption properties could offer unique opportunities to create new applications and fully utilize solar energy.

Figure 3

Field distributions on the B-Au excited at (a) 514 nm, (b) 532 nm, (c) 633 nm, (d) 694 nm, (e) 785 nm, (f) 1094 nm, and (g) 1150 nm simulated using three-dimensional FDTD (3D-FDTD) method and (h) the corresponding maximum field enhancement.

With strong broad-band absorption properties, the B-Au shows great potential for SERS because it is an optical phenomenon caused by the strong field enhancement effect associated with plasmon.[1] We have used two molecules, R6G with resonance Raman effect (to 514.5 nm excitation) and 4-MPY without resonance effect, to test the SERS performance of the B-Au. Figure shows the normalized SERS spectra for Rhodamine B (R6G) on Au nanodisk array (fabricated using electron-beam lithography[28]) and the B-Au excited at 532, 633, and 785 nm. The control Au nanodisk array shows optimized SERS at 785 nm excitation, while their SERS activity is rather weak at 532 and 633 nm excitation. The optimized SERS is achieved because the Au nanodisk array supports a narrow plasmon in resonance with the 785 nm laser (Figure S2), enabling the effective excitation of the plasmon. Clearly, achieving highly sensitive SERS signals on single, isolated plasmonic materials requires choosing an appropriate excitation light in resonance with their narrow plasmon.[29] In contrast, the B-Au produces strong SERS signals regardless of the excitation light used. This is because the B-Au supports multiple plasmons that can be excited by light over a wide spectral window (Figure ). For 532, 633, and 785 nm excitation, the maximum theoretical enhancement factor (EF ≈ M4) achieved is 5.95 × 103, 8.5 × 107, and 2.3 × 108 for the B-Au, respectively, while it is 2.1 × 102, 4.0 × 101, and 1.13 × 103 for the Au nanodisk (estimated from its field shown in Figure S2), respectively. We have further measured the SERS performance of the B-Au for 4-MPY (Figure S4). The SERS signal on the B-Au is still much stronger than that on the Au nanodisk array at 633 and 785 nm excitations. However, without resonance, Raman contribution to the SERS is not strong at 532 nm excitation on both the B-Au and Au nanodisk array. This is because at 532 nm excitation, the electrons in Au shows interband transition, which greatly attenuates the SERS effect. Outside the interband transition region, the B-Au still exhibits strong SERS behavior for different molecules (e.g., R6G and 4-MPY) over a wide excitation range (at least from 633 to 785 nm excitation and beyond). Therefore, the B-Au can produce broad-band SERS, which largely simplifies the choosing of excitation light for effective SERS, offering a great opportunity for easy, rapid SERS detection.

Figure 4

Normalized SERS spectra of R6G on (a) Au nanodisk array and (b) B-Au excited at different wavelengths , along with their corresponding SEM images (insets). All of the spectra were normalized by the intensity of the 520 cm–1 of a Si wafer measured at the same excitation power and for the same accumulation time (Figure S3).

Conclusions

In summary, we have shown that the adverse overcurrent disposition strategy in the electroplating industry can be exploited for the fabrication of fractal, plasmonic black gold (B-Au). With a unique fractal structure, the B-Au can support multiple plasmons, resulting in broad-band absorption properties. As a result, the B-Au shows interesting broad-band SERS properties. Beyond this, we believe the efficient broad-band absorption properties of the B-Au can also open up many other interesting applications for improved photo-electrocatalysis, photovoltaic, or photothermal energy conversion, which are not achievable or are of low efficiency on the materials with narrowband plasmons.

Materials and Methods

Preparation of the B-Au

The B-Au was deposited at room temperature and a constant current of 3.2 mA/cm2 for 800 s in a traditional three-electrode electrochemical cell. Gold film (50 nm)-coated glass slide, platinum plate (2.0 × 2.0 cm2), and saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. Gold plating solution (ECF-88) was purchased from Metalor Technologies (Neuchatel, Switzerland).

Preparation of the Gold Nanodisk Arrays

A 150 nm thick positive-tone electron-beam resist PMMA (950 K, 3 wt % in anisole) was spin-coated onto a SiO2/Si substrate at 1000 rpm and then the substrate was baked at 180 °C for 120 s. Subsequently, a Raith150 two electron-beam lithography (EBL) (Raith, Germany) system was used for the exposure with an accelerating voltage of 30 kV, a beam current of 230 pA, and an exposure dose of 800 μC/cm2. After exposure, the sample was developed in a mixture of methyl isobutyl ketone and isopropyl alcohol (IPA) (volume ratio 1:3) at −18 °C for 60 s and then immersed in IPA solution for 30 s to stop the development. The samples were finally blow-dried with a steady N2 blow.

The samples were then metallized using a thermal evaporation system (JSD300, Anhui JiashuoVacuum Technology Co. Ltd.). Before deposition, the chamber was evacuated to a pressure of 1.0 × 10–5 Pa. A 1 nm Cr adhesion layer was first deposited to ensure good adhesion, followed by the deposition of a 30 nm Au film. To improve the deposition quality, the working pressure and the temperature of the chamber were, respectively, kept at constant values of 4 × 10–4 Pa and 14 °C during the whole evaporation process. The thickness of the deposited Au film was monitored using an angstrom-sensitivity quartz-crystal microbalance. After the deposition of the Au film, a lift-off process was conducted in acetone solution with ultrasonic agitation for 120 s and then the sample was immersed in the IPA solution for 30 s, followed by drying with a steady N2 blow.

Spectral Measurements and Thermal Imaging

Absorption spectra of the Au deposits were measured on a UV-3600Plus UV–vis–NIR spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere. The samples were placed at the side of the integrated sphere, and light was incident with an angle of 8 degrees. Transmission (T) and reflection (R) were collected by the integrating sphere detector with all ports closed except the one for the incident beam. For an empty integrating sphere and the B-Au (deposited on Au film-coated glass slide), their transmission measured is 100% and 0, respectively. Therefore, the absorption of the B-Au can be determined by subtracting the transmission and reflection portions from the overall incident light (i.e., A = 100% – T – R = 100% – R). SERS measurements were measured on an invia-Reflex micro-Raman spectrometer (Renishaw, U.K.) using 532, 633, and 785 nm laser as the excitation sources. Before SERS measurements, the B-Au was immersed in a 1 × 10–5 M R6G solution (or a 1 mM 4-mercappyridine (4-MPY) solution) for about 5 minutes, removed from the solution, and then dried naturally in the air. To avoid burning the molecule, the laser intensity was attenuated to 10% and all the spectra were measured for a single 1 s accumulation. Because the surface of the B-Au is less uniform, each SERS spectrum offered was averaged from three tests.

Thermal imaging of the B-Au and temperature change curve during light illumination is obtained from a Fotric 222s infrared camera (Fotric, America). During these tests, the B-Au was vertically illuminated by light from a Xenon lamp at a power density of 100 mW/cm2.

FDTD Simulations

Reflection spectra of the B-Au and field distributions on the B-Au were simulated using a 3-dimensional finite-difference time-domain (3D-FDTD) numerical method. All of the simulations were performed on commercial software (Lumerical Solutions, Canada). To avoid unnecessary boundary reflections around structures and to match the fabricated structure, the simulations adopted perfectly matched layer conditions at the three axes. The excitation light was a linearly polarized plane-wave light normally incident on the B-Au. The simulation time was set to be 1000 fs to guarantee convergence. To save computation time and guarantee the accuracy, an nonuniform mesh was employed, in which the Yee cells of 2 × 2 × 2 nm3 and 4 × 4 × 4 nm3 were used for the slit region and other regions, respectively. The dielectric function of gold was taken from a multicoefficient fitting model offered by FDTD software. The reflectivity (R) is the ratio of the time-averaged power across a reflective surface and time-averaged power of the incident source. By knowing R, the absorption of the B-Au with a nearly zero transmission as well can be given by A = 100% – R. The electric field enhancement is defined as lg (Eloc/Ein)4, where Eloc and Ein are the magnitudes of the localized and incident fields, respectively.