**2. Fabrication ofsilver nanostructure as sensitive SERS substrates**

The detection sensitivity of SERS depends considerably on the surface property of the SERS substrate. High aspect ratio, nanostructured Ag, Au, Cu substrates are proved to be good SERS substrates. For instance, using ordered arrays of gold particles prepared through a po‐ rous alumina template as the SERS substrate, Rhodamine 6G (R6G) molecules were detected to a concentration limit of 10-12 M; arrays of silicon nanorods coated with thin films of Ag served as good SERS substrates for R6G molecule detection, etc [12, 16]. Thus the prepara‐ tion of SERS substrates with preferred surface property is of great importance. There are several methods to prepare these kinds of SERS substrates and in this chapter we take glanc‐ ing angle deposition as an example.

Glancing angle deposition (GLAD) technique is a simple but powerful means of producing thin films with pre-designed nanostructures, such as nanopillars, slanted posts, zigzag col‐ umns and spirals. Silver nanorod arrays prepared by GLAD are excellent SERS substrates.

In addition, the SERS properties are related to the optical properties of the nanorod arrays. Both SERS properties and optical properties depend on the structure of the nanorods, such as the shape, length, separation, tilting angle and so on, which can be tuned by the deposi‐ tion conditions.

#### **2.1. Fabrication of sensitive SERS substrates by GLAD**

The detection sensitivity of the SERS technique depends greatly on the surface property of the SERS substrate [45,46]. Among the approaches so far available to prepare nanostruc‐ tured materials, the glancing angle deposition (GLAD) technique is a simple but powerful means of producing thin films with pre-designed nanostructures [47-48], such as nanopil‐ lars, slanted posts, zigzag columns, spirals [18-19], etc [20-23]. For example, arrays of Ag nanorods were found to be good SERS substrates for the detection of trans-1,2-bis(4-pyrid‐ yl)ethane molecules, with a SERS enhancement factor greater than 108 [16]. It is therefore of great interest to investigate the growth of metal nanostructures by the GLAD technique [49].

Pristine Si wafers with (001) orientation were used as substrates. These were supersonically cleaned in acetone, ethanol and de-ionized water baths in sequence, and were fixed on the GLAD substrate in an e-beam deposition system. The system was pumped down to a vac‐ uum level of 3×10-5 Pa and then the thin Ag film was deposited on the substrate with a de‐ positing rate of 0.5 nm/s, with the thickness monitored by a quartz crystal microbalance. To produce films of aligned Ag nanorods, the incident beam of Ag flux was set at ~ 85 o from the normal of the silicon substrate, at different substrate temperatures. The morphology and structure of the thin Ag films was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and high-resolution TEM, selected area diffraction (SAD) and X-ray diffraction (XRD), respectively. The performance of the nanostructured Ag films as SERS substrates was evaluated with a micro-Raman spectrometer using R6G as the model molecule.

used in the field of SERS. For instance, using Ag nanorods as SERS substrates,Rhodamine 6G with concentration of 10-14 M (dissolved in water) was detected [9]; with the aluminamodified AgFON substrates, bacillus subtilis spores were detected to 10-14 M [10, 11]; Vo-Dinh reported even the detection of specific nucleic acid sequences by the SERS technique [12-14]. In spite of the numerous studies on the application as a chemical and biological sen‐ sor [15-17], the SERS technique has not yet been employed to detect PCBs as they are hardly

**2. Fabrication ofsilver nanostructure as sensitive SERS substrates**

The detection sensitivity of SERS depends considerably on the surface property of the SERS substrate. High aspect ratio, nanostructured Ag, Au, Cu substrates are proved to be good SERS substrates. For instance, using ordered arrays of gold particles prepared through a po‐ rous alumina template as the SERS substrate, Rhodamine 6G (R6G) molecules were detected to a concentration limit of 10-12 M; arrays of silicon nanorods coated with thin films of Ag served as good SERS substrates for R6G molecule detection, etc [12, 16]. Thus the prepara‐ tion of SERS substrates with preferred surface property is of great importance. There are several methods to prepare these kinds of SERS substrates and in this chapter we take glanc‐

Glancing angle deposition (GLAD) technique is a simple but powerful means of producing thin films with pre-designed nanostructures, such as nanopillars, slanted posts, zigzag col‐ umns and spirals. Silver nanorod arrays prepared by GLAD are excellent SERS substrates.

In addition, the SERS properties are related to the optical properties of the nanorod arrays. Both SERS properties and optical properties depend on the structure of the nanorods, such as the shape, length, separation, tilting angle and so on, which can be tuned by the deposi‐

The detection sensitivity of the SERS technique depends greatly on the surface property of the SERS substrate [45,46]. Among the approaches so far available to prepare nanostruc‐ tured materials, the glancing angle deposition (GLAD) technique is a simple but powerful means of producing thin films with pre-designed nanostructures [47-48], such as nanopil‐ lars, slanted posts, zigzag columns, spirals [18-19], etc [20-23]. For example, arrays of Ag nanorods were found to be good SERS substrates for the detection of trans-1,2-bis(4-pyrid‐ yl)ethane molecules, with a SERS enhancement factor greater than 108 [16]. It is therefore of great interest to investigate the growth of metal nanostructures by the GLAD technique [49].

Pristine Si wafers with (001) orientation were used as substrates. These were supersonically cleaned in acetone, ethanol and de-ionized water baths in sequence, and were fixed on the GLAD substrate in an e-beam deposition system. The system was pumped down to a vac‐ uum level of 3×10-5 Pa and then the thin Ag film was deposited on the substrate with a de‐

dissolved in water.

30 Organic Pollutants - Monitoring, Risk and Treatment

ing angle deposition as an example.

**2.1. Fabrication of sensitive SERS substrates by GLAD**

tion conditions.

It is well known that the major factors influencing the growth morphology of the films by GLAD are the incident direction of the depositing beam flux, the temperature and the move‐ ment of the substrate, and the deposition rate, etc. When fixing the incident Ag flux at ~85 o from the normal of the substrate and the deposition rate at ~0.5 nm/s, the growth morpholo‐ gy of the Ag films was greatly dependent on the temperature and movement of the sub‐ strate. Fig 1 shows the growth morphology of thin Ag films versus the temperature and movement of the substrate. The SEM micrographs were taken by a FEI SEM (QUANTA 200FEG) working at 20 kV.

Fig 1(a) and (b) shows typical SEM images of the surface morphology of thin Ag films deposited at 120 o C, without substrate rotation and with substrate rotation at a speed of 0.2 rpm, respectively. One sees from the images that at this temperature, Ag nanorods formed in two films with a length of 500 nm, yet they were not well separated - most nanorods were joined together. A major difference between the two is the growth direc‐ tion of the joined nanorods, i.e. without rotation the nanorods grew at a glancing angle on the substrate, while with substrate rotation the nanorods grew vertically aligned. An‐ other difference noticeable is the size of the nanorods, i.e. nanorods grown with substrate rotation have a slightly larger diameter.

Fig 1(c) and (d) shows respectively the surface morphology of thin Ag films deposited at -40 o C, without substrate rotation and with rotation at a speed of 0.2 rpm. Comparing with Figs 1(a) and (b), it can be seen that the decrease in the deposition temperature led to the separa‐ tion of Ag nanorods in the two films, while the rotation of the substrate also determined the growth direction and diameter of the nanorods, as observed from Figs 1(a) and 1(b). The Ag nanorods grown at this temperature are 20-30 nm in diameter, ~ 800 nm in length and are well separated. Therefore, through adjusting the temperature and movement of the sub‐ strate one can grow well separated and aligned Ag nanorods on planar silicon substrates.

Fig 1(e) and 1(f) shows respectively a bright-field TEM and a HRTEM image of Ag nanorods shown by Fig 1(c); inset of Fig 1(f) is the corresponding SAD pattern. The images and the SAD pattern were taken with a JEM-2011F working at 200 kV. One sees from the Figs that the Ag nanorod is ~ 30 nm in diameter and its micro-structure is single crystalline. By index‐ ing the SAD pattern it is noticed that during the growth process the {111} plane of the nano‐ rod was parallel to the substrate surface, with its axis along the <110> direction. This was confirmed by XRD analysis. Fig 2 shows a XRD pattern of the Ag nanorods shown by Fig 1(c). The pattern was taken with a Rigaku X-ray diffractometer using the Cu k line, work‐ ing at the θ-2θ coupled scan mode. From the Fig, a very strong (111) texture is observed, indicating that the {111} plane of the Ag nanorods was parallel to the substrate surface. These suggest that one can produce arrays of aligned, single crystalline Ag nanorods by the GLAD technique even at a low substrate temperature, i.e. -40 o C.

**Figure 2.** A XRD pattern of the Ag film consisting of well separated, single crystalline nanorods shown by Fig 1(c).

nanorods represent excellent SERS performance.

tration limit of this method is 10-14mol/L in the authors' work.

By using Rhodamine 6G as the model molecule, the performance of thin Ag films shown by Figs 1(a)-(d) is examined as the SERS substrates. These samples were dipped in a 1×10-6mol/L solution of R6G in water for 30 minutes and dried with a continuous gentle ni‐ trogen blow. Fig 3(a) and 3(b) show Raman spectra of R6G obtained on the four nanostruc‐ tured Ag films by a Reinshaw 100 Raman spectrometer using a 514 nm Ar+ laser as the excitation source. It is observed that with the thin Ag films as the SERS substrate, all spectra exhibit clearly the characteristic peaks of R6G molecules, at 612, 774, 1180, 1311, 1361, 1511, 1575 and 1648 cm-1, respectively12.However, the intensity of the Raman peaks was depend‐ ent on the morphology of the films. It is noticed that on Ag films consisting of well separat‐ ed nanorods, see Figs 3(b), the Raman peaks of R6G are much stronger than those on films of joined nanorods, see Figs 3(a). This suggests that arrays of aligned but well separated Ag

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Using arrays of aligned Ag nanorods shown by Figs 1(c) and 1(d) as SERS substrates, we examined the detection limit of R6G molecules in water by the SERS technique. Fig 4(a) shows Raman spectra of R6G obtained on Ag nanorods shown by Fig 1(c), as a function of the concentration of R6G in water ranging from 1×10-8 to 1×10-16mol/L. Similar results were also obtained for Ag nanorods shown by Fig 1(d). The Raman spectra were obtained by one scan with an accumulation time of 10 s, at a laser power of 1 % to avoid decomposition of R6G. It is found that characteristic peaks of R6G were all observed at all concentrations. To clearly show this, we plot the Raman spectrum at 10-14mol/L in Fig 4(b). It is noticed that al‐ though the intensity of the peaks is almost two orders lower than that at 10-6mol/L, the spec‐ trum contains the clear characteristic peaks of R6G12. These suggest that Ag films consisting of aligned and well separated Ag nanorods with single crystalline could serve as excellent SERS substrate for trace amount detection of R6G molecules. However, in the Raman spec‐ trum at 10-16mol/L in Fig 4(a), some of the peaks of R6G disappear. That suggests the concen‐

**Figure 1.** Growth morphology of thin Ag films by GLAD at various conditions. (a) at 120 oC without substrate rotation; (b) at 120 oC and substrate rotation at 0.2 rpm; (c) at -40 oC without substrate rotation; and (d) at -40 oC and substrate rotation at 0.2 rpm. (e) and (f) shows respectively a bright-field TEM and a HRTEM image of the nanorods shown by Fig 1(c); inset of (f) is the corresponding SAD pattern.

indicating that the {111} plane of the Ag nanorods was parallel to the substrate surface. These suggest that one can produce arrays of aligned, single crystalline Ag nanorods by the

**Figure 1.** Growth morphology of thin Ag films by GLAD at various conditions. (a) at 120 oC without substrate rotation; (b) at 120 oC and substrate rotation at 0.2 rpm; (c) at -40 oC without substrate rotation; and (d) at -40 oC and substrate rotation at 0.2 rpm. (e) and (f) shows respectively a bright-field TEM and a HRTEM image of the nanorods shown by

Fig 1(c); inset of (f) is the corresponding SAD pattern.

C.

GLAD technique even at a low substrate temperature, i.e. -40 o

32 Organic Pollutants - Monitoring, Risk and Treatment

**Figure 2.** A XRD pattern of the Ag film consisting of well separated, single crystalline nanorods shown by Fig 1(c).

By using Rhodamine 6G as the model molecule, the performance of thin Ag films shown by Figs 1(a)-(d) is examined as the SERS substrates. These samples were dipped in a 1×10-6mol/L solution of R6G in water for 30 minutes and dried with a continuous gentle ni‐ trogen blow. Fig 3(a) and 3(b) show Raman spectra of R6G obtained on the four nanostruc‐ tured Ag films by a Reinshaw 100 Raman spectrometer using a 514 nm Ar+ laser as the excitation source. It is observed that with the thin Ag films as the SERS substrate, all spectra exhibit clearly the characteristic peaks of R6G molecules, at 612, 774, 1180, 1311, 1361, 1511, 1575 and 1648 cm-1, respectively12.However, the intensity of the Raman peaks was depend‐ ent on the morphology of the films. It is noticed that on Ag films consisting of well separat‐ ed nanorods, see Figs 3(b), the Raman peaks of R6G are much stronger than those on films of joined nanorods, see Figs 3(a). This suggests that arrays of aligned but well separated Ag nanorods represent excellent SERS performance.

Using arrays of aligned Ag nanorods shown by Figs 1(c) and 1(d) as SERS substrates, we examined the detection limit of R6G molecules in water by the SERS technique. Fig 4(a) shows Raman spectra of R6G obtained on Ag nanorods shown by Fig 1(c), as a function of the concentration of R6G in water ranging from 1×10-8 to 1×10-16mol/L. Similar results were also obtained for Ag nanorods shown by Fig 1(d). The Raman spectra were obtained by one scan with an accumulation time of 10 s, at a laser power of 1 % to avoid decomposition of R6G. It is found that characteristic peaks of R6G were all observed at all concentrations. To clearly show this, we plot the Raman spectrum at 10-14mol/L in Fig 4(b). It is noticed that al‐ though the intensity of the peaks is almost two orders lower than that at 10-6mol/L, the spec‐ trum contains the clear characteristic peaks of R6G12. These suggest that Ag films consisting of aligned and well separated Ag nanorods with single crystalline could serve as excellent SERS substrate for trace amount detection of R6G molecules. However, in the Raman spec‐ trum at 10-16mol/L in Fig 4(a), some of the peaks of R6G disappear. That suggests the concen‐ tration limit of this method is 10-14mol/L in the authors' work.

**Figure 3.** Raman spectra of R6G on thin Ag films consisting of (a) joined nanorods shown by Figs 1(a) (black line) and 1(b) (grey line); and (b) separated Ag nanorods shown by Figs 1(c) (black line) and 1(d) (grey line), respectively, at a concentration of 1×10-6mol/L.

**Figure 4.** a) Raman spectra of R6G at concentrations ranging from 1×10-8 to 1×10-16mol/L; and (b) the Raman spec‐ trum of R6G at a concentration of 1×10-14mol/L, on the thin Ag film consisting of well separated, single crystalline Ag

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Although the Ag nanorod arrays present sensitive SERS performance, it is still necessary to enable the substrate to detect organic pollutants at trace amount with adequate sensitivity.

There are several ways to promote the sensitivity of Ag nanorods as SERS substrates.

**2.2. Enhancement of the sensitivity of SERS substrates via underlayer films**

nanorods.

(a) (a)

34 Organic Pollutants - Monitoring, Risk and Treatment

concentration of 1×10-6mol/L.

(b) (b)

**Figure 3.** Raman spectra of R6G on thin Ag films consisting of (a) joined nanorods shown by Figs 1(a) (black line) and 1(b) (grey line); and (b) separated Ag nanorods shown by Figs 1(c) (black line) and 1(d) (grey line), respectively, at a

(c) (c)

**Figure 4.** a) Raman spectra of R6G at concentrations ranging from 1×10-8 to 1×10-16mol/L; and (b) the Raman spec‐ trum of R6G at a concentration of 1×10-14mol/L, on the thin Ag film consisting of well separated, single crystalline Ag nanorods.

#### **2.2. Enhancement of the sensitivity of SERS substrates via underlayer films**

Although the Ag nanorod arrays present sensitive SERS performance, it is still necessary to enable the substrate to detect organic pollutants at trace amount with adequate sensitivity. There are several ways to promote the sensitivity of Ag nanorods as SERS substrates.

Much effort has been devoted to achieving highly sensitive SERS substrates. In particular, multilayer structures can improve SERS enhancement, such as "sandwich" structures with silver oxide or carbon inside and Ag or Au as both underlayer and overlayer [24-28]. Other researchers found that multilayer structures of Ag/Au nanostructures on the smooth metal‐ lic underlayer exhibited better SERS sensitivity compared to those without metallic under‐ layer (EF = 5 × 108 ) [29-31]. However, the factor that governs the enhancement for multilayer structures is not very clear. Recently, Misra*et al*. obtained remarkably high SERS sensitivity using a micro-cavity with a radius of several micrometers [32]. Shoute*et al*. obtained high SERS signals (EF = 6 × 106 ) for molecules adsorbed on the silver island films supported by thermally oxidized silicon wafers and declared that the additional enhancement was due to the optical interference effect [33]. All the above experiments and those conducted by Dris‐ kell*et al*. suggested that the underlayer reflectivity could play an important role in the multi‐ layer SERS substrates [29].

in general, *R* Ag>*R* Al>*R* Ti>*R* Si. In the visible wavelength region, the reflectivity of Ag, Al and

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37

**Figure 5.** a) Calculated reflectivity *R* of thin Ag, Al, Si and Ti films at different wavelengths λ with film thickness of 100 nm; b) calculated reflectivity *R* of thin Ag, Al, Si and Ti films with different thicknesses *d* at λ <sup>0</sup> = 785 nm; c) experimen‐

The twelve deposited planar thin film samples were then loaded into another custom-de‐ signed electron-beam evaporation system for Ag nanorod deposition through the so-called oblique angle deposition (OAD) [16, 29, 37].In this deposition, the background pressure was 1×10-7Torr and the substrate holder was rotated so that the deposition flux was incident onto the thin films with an angle *θ* = 86º with respect to the surface normal of the substrate hold‐ er. The Ag nanorod arrays were formed through a self-shadowing effect [16, 29, 37]. During the deposition, the Ag deposition rate was monitored by a QCM directly facing the incident vapour. The deposition rate was fixed at 0.3 nm/s and the deposition ended when the QCM

The morphologies of the Ag nanorod arrays on different thin film substrates were character‐ ized by a scanning electron microscope (SEM, FEI Inspect F). The typical top-view SEM im‐

tally obtained reflectivity spectra of thin Ag, Al, Si and Ti films with different thicknesses.

read 2000 nm (our optimized condition).

Ti increases with the thickness *d*, while Si demonstrates an oscillating behaviour.

We have investigated in detail the relationship of underlayer reflectivity and the SERS en‐ hancement of Ag nanorod substrates prepared by oblique angle deposition. We use thin films of different materials with different thicknesses as underlayers to modulate the reflec‐ tivity systematically. With the coating of the same Ag nanorods, we find that the SERS in‐ tensity increases linearly with the underlayer reflectivity. This conclusion can be explained by a modified Greenler's model we recently developed [34].

To change the reflectivity of the underlayer films, one can vary the dielectric constant and the thickness of the films systematically. We proposed to use Ag, Al, Si and Ti films, since they have different dielectric constants and can be fabricated easily. With a transfer matrix method, we can calculate the reflectivity of those films [35, 36].Fig 5(a) shows the calculated reflectivity spectra of 100 nm Ag, Al, Si, and Ti films. In general, the reflectivity, *R* Ag>*R* Al>*R* Ti>RSi, except that at *λ* ~ 600 nm where the Si film has a large constructive interference. Fig 5(b) plots the film thickness dependent reflectivity for Ag, Al, Si and Ti at a fixed wave‐ length *λ* 0 = 785 nm. The reflectivity of Ag, Al, Ti, e.g. metals, increases monotonically with the film thickness *d*. The reflectivity *R* of Ag, Al and Ti thin films increases sharply when *d*< 100 nm and almost remains unchanged when 100 nm ≤ *d* ≤ 400 nm, while *R* Si shows an oscil‐ lative behaviour due to the interference effect of a dielectric layer.

We deposited thin Ag, Al, Si and Ti films, all with thickness *d* = 25, 100, and 400 nm, respec‐ tively, to achieve different reflectivity. All depositions were carried out in a custom-de‐ signed electron-beam deposition system [16].Before the deposition, the glass slide substrates were cleaned by piranha solution (H2SO4 : H2O2 = 4:1 in volume). The pellets of source mate‐ rials, Ag, Al, Ti, with 99.99% purity, were purchased from Kurt J. Lesker Company, and Si with 99.9999% purity was purchased from Alfa Aesar Company. The film thickness was monitored *in situ* by a quartz crystal microbalance (QCM) facing toward the vapour source. After the deposition, the reflectivity of the deposited thin films was measured by an Ultra‐ violet-Visible Spectrophotometer (UV-Vis) double beam spectrophotometer with an inte‐ grating sphere (Shimadzu UV-Vis 2450). Fig 5(c) shows the reflectivity spectra of the twelve thin films obtained. The shapes of the reflection spectra are qualitatively consistent with those predicted by the calculations, as shown in Figs. 5(a) and (b). At the same wavelength, in general, *R* Ag>*R* Al>*R* Ti>*R* Si. In the visible wavelength region, the reflectivity of Ag, Al and Ti increases with the thickness *d*, while Si demonstrates an oscillating behaviour.

Much effort has been devoted to achieving highly sensitive SERS substrates. In particular, multilayer structures can improve SERS enhancement, such as "sandwich" structures with silver oxide or carbon inside and Ag or Au as both underlayer and overlayer [24-28]. Other researchers found that multilayer structures of Ag/Au nanostructures on the smooth metal‐ lic underlayer exhibited better SERS sensitivity compared to those without metallic under‐

structures is not very clear. Recently, Misra*et al*. obtained remarkably high SERS sensitivity using a micro-cavity with a radius of several micrometers [32]. Shoute*et al*. obtained high

thermally oxidized silicon wafers and declared that the additional enhancement was due to the optical interference effect [33]. All the above experiments and those conducted by Dris‐ kell*et al*. suggested that the underlayer reflectivity could play an important role in the multi‐

We have investigated in detail the relationship of underlayer reflectivity and the SERS en‐ hancement of Ag nanorod substrates prepared by oblique angle deposition. We use thin films of different materials with different thicknesses as underlayers to modulate the reflec‐ tivity systematically. With the coating of the same Ag nanorods, we find that the SERS in‐ tensity increases linearly with the underlayer reflectivity. This conclusion can be explained

To change the reflectivity of the underlayer films, one can vary the dielectric constant and the thickness of the films systematically. We proposed to use Ag, Al, Si and Ti films, since they have different dielectric constants and can be fabricated easily. With a transfer matrix method, we can calculate the reflectivity of those films [35, 36].Fig 5(a) shows the calculated reflectivity spectra of 100 nm Ag, Al, Si, and Ti films. In general, the reflectivity, *R* Ag>*R* Al>*R* Ti>RSi, except that at *λ* ~ 600 nm where the Si film has a large constructive interference. Fig 5(b) plots the film thickness dependent reflectivity for Ag, Al, Si and Ti at a fixed wave‐ length *λ* 0 = 785 nm. The reflectivity of Ag, Al, Ti, e.g. metals, increases monotonically with the film thickness *d*. The reflectivity *R* of Ag, Al and Ti thin films increases sharply when *d*< 100 nm and almost remains unchanged when 100 nm ≤ *d* ≤ 400 nm, while *R* Si shows an oscil‐

We deposited thin Ag, Al, Si and Ti films, all with thickness *d* = 25, 100, and 400 nm, respec‐ tively, to achieve different reflectivity. All depositions were carried out in a custom-de‐ signed electron-beam deposition system [16].Before the deposition, the glass slide substrates were cleaned by piranha solution (H2SO4 : H2O2 = 4:1 in volume). The pellets of source mate‐ rials, Ag, Al, Ti, with 99.99% purity, were purchased from Kurt J. Lesker Company, and Si with 99.9999% purity was purchased from Alfa Aesar Company. The film thickness was monitored *in situ* by a quartz crystal microbalance (QCM) facing toward the vapour source. After the deposition, the reflectivity of the deposited thin films was measured by an Ultra‐ violet-Visible Spectrophotometer (UV-Vis) double beam spectrophotometer with an inte‐ grating sphere (Shimadzu UV-Vis 2450). Fig 5(c) shows the reflectivity spectra of the twelve thin films obtained. The shapes of the reflection spectra are qualitatively consistent with those predicted by the calculations, as shown in Figs. 5(a) and (b). At the same wavelength,

by a modified Greenler's model we recently developed [34].

lative behaviour due to the interference effect of a dielectric layer.

) [29-31]. However, the factor that governs the enhancement for multilayer

) for molecules adsorbed on the silver island films supported by

layer (EF = 5 × 108

SERS signals (EF = 6 × 106

36 Organic Pollutants - Monitoring, Risk and Treatment

layer SERS substrates [29].

**Figure 5.** a) Calculated reflectivity *R* of thin Ag, Al, Si and Ti films at different wavelengths λ with film thickness of 100 nm; b) calculated reflectivity *R* of thin Ag, Al, Si and Ti films with different thicknesses *d* at λ <sup>0</sup> = 785 nm; c) experimen‐ tally obtained reflectivity spectra of thin Ag, Al, Si and Ti films with different thicknesses.

The twelve deposited planar thin film samples were then loaded into another custom-de‐ signed electron-beam evaporation system for Ag nanorod deposition through the so-called oblique angle deposition (OAD) [16, 29, 37].In this deposition, the background pressure was 1×10-7Torr and the substrate holder was rotated so that the deposition flux was incident onto the thin films with an angle *θ* = 86º with respect to the surface normal of the substrate hold‐ er. The Ag nanorod arrays were formed through a self-shadowing effect [16, 29, 37]. During the deposition, the Ag deposition rate was monitored by a QCM directly facing the incident vapour. The deposition rate was fixed at 0.3 nm/s and the deposition ended when the QCM read 2000 nm (our optimized condition).

The morphologies of the Ag nanorod arrays on different thin film substrates were character‐ ized by a scanning electron microscope (SEM, FEI Inspect F). The typical top-view SEM im‐

ages are shown in Fig. 6 and they all look very similar. From the cross-section and top-view SEM images, the length *L*, diameter *D* and separation *S* of these Ag nanorods on different planar thin films are obtained statistically: *L* Ag= 940 ± 70 nm, *D* Ag= 90 ± 10 nm, *S* Ag= 140 ± 30 nm; *L* Al= 950 ± 50 nm, *D* Al= 90 ± 10 nm, *S* Al= 140 ± 30 nm; *L* Si= 900 ± 50 nm, *D* Si= 80 ± 10 nm, *S* Si= 130 ± 20 nm; and *L* Ti= 930 ± 60 nm, *D* Ti= 90 ± 10 nm, *S* Ti= 130 ± 20 nm, respectively. The Ag nanorod tilting angles *β* were measured to be about 73ºwith respect to substrate normal, which are consistent with our previous results [16, 29, 37]. These structure parameters are very close to one another, implying that the Ag nanorod arrays deposited on different thin film substrates are statistically the same. The SERS response of these Ag nanorod substrates were evaluated under identical conditions: A 2 μL droplet of a Raman probe molecule, trans-1, 2- bis (4-pyridyl) ethylene (BPE) with a concentration of 10-5 M, was uniformly dis‐ persed onto the Ag nanorod substrates. The SERS spectra were recorded by the HRC-10HT Raman Analyzer from EnwaveOptronics Inc., with an excitation wavelength of *λ* 0 = 785 nm, a power of 30 mW and an accumulation time of 10 s.

can be assigned to the C=C stretching mode, aromatic ring stretching mode and in-plane ring mode, respectively [38]. The SERS intensity of the Ag nanorods grown on thin Ag film are higher than others and the SERS intensity of the Ag nanorods on Al film is larger than that on Ti film. The Ag nanorods on Si film show the smallest SERS intensity. According to Fig. 5(c), this seems to follow a trend: the larger the underlayer reflectivity, the larger the SERS intensity. To quantitatively compare the SERS response of these substrates, the Raman

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Fig 7(b) plots the SERS intensity *I* 1200 versus the reflectivity *R* of the underlayer thin films at *λ* 0 = 785 nm. The error bar for the Raman intensity is the standard deviation from 15 or more measurements from multiple sampling spots on the same substrates and the error bar for the reflectivity data is calculated from multiple reflectivity measurements at *λ* 0 = 785 nm. In Fig. 7(b), the SERS intensity and reflectivity follow a linear relationship: when the reflec‐

This linear relationship of the underlayer reflectivity and SERS intensity can be explained by a modified Greenler's model developed by Liu *et al*. [34]Greenler's model is proposed through classical electrodynamics to explain the effects of the incident angles and polari‐ zation, and the collecting angle on the Raman scattering from a molecule adsorbed on a planar surface [32]. The modified Greenler's model extended the Greenler's model from a planar surface to Ag nanorod substrates and considered the effect of the underlying sub‐ strate [34]. The main point of the modified Greenler's model is to consider the conditions of both the incident and scattering fields near the molecule absorbed on a nanorod to cal‐

Assuming that relative Raman intensity *η* is the ratio of the total Raman scattering power to incident light power, according to the modified Greenler's model, *η* excited by an unpolar‐

where *R* p and *R* s are the reflectivity of *p*- and *s*-polarized lights by the Ag nanorod surface, and *R'* p and *R'* <sup>s</sup> are the reflectivity of *p*- and *s*-polarized components by the underlayer thin film; *n* 2 is complex refractive index of Ag, and *n* 2 = 0.03 + 5.242*i* (for *λ* <sup>0</sup> = 785 nm); *φ* is the light incident angle, and *β* is the Ag nanorod tilting angle; *Δ* = *d* (1 + *cos* 2*φ*) / *cosφ*, where *d* is the thickness of Ag nanorod layer; *δ* p, *δ* s, *δ'* p, and *δ'* s are the reflectivity phase shifts of *p*-

*<sup>s</sup>* )/ Re(*r*'

*s* )

+ 2*πΔ* / *λ*)sin2*β*

(*φ* −*β*)

<sup>+</sup> <sup>2</sup> *Rp*cos*δp*)cos<sup>2</sup>

2 *R*' *<sup>p</sup>* 1 <sup>2</sup> cos(*δ*' *<sup>p</sup>*

and *s*-polarization E-fields from Ag nanorods and underlayer thin film, defined as

*<sup>s</sup>* =tan−<sup>1</sup> Im(*r*'

+ 2*πΔ* / *λ* −*δp*) (1 + *Rp*

*<sup>s</sup>* + 2*πΔ* / *λ*) }

tivity of the underlayer increases, the SERS enhancement factor increases.

peak intensity *I* <sup>1200</sup> at Δ*ν* = 1200 cm-1 is analyzed.

culate the enhancement.

*<sup>η</sup>* <sup>=</sup> <sup>&</sup>lt;*ERaman* <sup>2</sup> <sup>&</sup>gt; / <sup>&</sup>lt;*Eincident*

+ *n*<sup>2</sup> 4

<sup>2</sup> sin2*φ*cos(*δ*' *<sup>p</sup>*

= 1

+2*n*<sup>2</sup> 2 *Rp* 1 <sup>2</sup> *R*' *<sup>p</sup>* 1

+ 1 + *n*<sup>2</sup> 4 *R*' *<sup>s</sup>* + 2*n*<sup>2</sup> 2 *R*' *s* 1 <sup>2</sup> cos(*δ*'

<sup>2</sup> { <sup>1</sup> <sup>+</sup> *Rp*

ized light can be explicitly expressed as [34]

<sup>2</sup> >

*<sup>δ</sup><sup>p</sup>* =tan−<sup>1</sup> Im(*rp*)/ Re(*rp*) , *<sup>δ</sup><sup>s</sup>* =tan−<sup>1</sup> Im(*rs*) / Re(*rs*)

*<sup>δ</sup>*' *<sup>p</sup>* =tan−<sup>1</sup> Im(*r*' *<sup>p</sup>* ) / Re(*r*' *<sup>p</sup>* ) , *<sup>δ</sup>*'

*R*' *<sup>p</sup>* cos*δp*cos2(*φ* −*β*) + 2*n*<sup>2</sup>

**Figure 6.** Representative SEM images of Ag nanorod arrays on 100 nm underlayer thin films with different materials: (a) Ag; (b) Al; (c) Ti; (d) Si. All the Figs have the same scale bar.

Fig 7(a) shows the representative BPE SERS spectra obtained at *λ* <sup>0</sup> = 785 nm from the Ag nanorod arrays on thin Ag, Al, Si and Ti film underlayers (thickness *d* = 100 nm). Each spec‐ trum is an average of at least 15 different spectra taken at different spots on the substrates. All of them show the three main Raman bands of BPE, Δ*ν* = 1639, 1610, and 1200 cm-1, which can be assigned to the C=C stretching mode, aromatic ring stretching mode and in-plane ring mode, respectively [38]. The SERS intensity of the Ag nanorods grown on thin Ag film are higher than others and the SERS intensity of the Ag nanorods on Al film is larger than that on Ti film. The Ag nanorods on Si film show the smallest SERS intensity. According to Fig. 5(c), this seems to follow a trend: the larger the underlayer reflectivity, the larger the SERS intensity. To quantitatively compare the SERS response of these substrates, the Raman peak intensity *I* <sup>1200</sup> at Δ*ν* = 1200 cm-1 is analyzed.

ages are shown in Fig. 6 and they all look very similar. From the cross-section and top-view SEM images, the length *L*, diameter *D* and separation *S* of these Ag nanorods on different planar thin films are obtained statistically: *L* Ag= 940 ± 70 nm, *D* Ag= 90 ± 10 nm, *S* Ag= 140 ± 30 nm; *L* Al= 950 ± 50 nm, *D* Al= 90 ± 10 nm, *S* Al= 140 ± 30 nm; *L* Si= 900 ± 50 nm, *D* Si= 80 ± 10 nm, *S* Si= 130 ± 20 nm; and *L* Ti= 930 ± 60 nm, *D* Ti= 90 ± 10 nm, *S* Ti= 130 ± 20 nm, respectively. The Ag nanorod tilting angles *β* were measured to be about 73ºwith respect to substrate normal, which are consistent with our previous results [16, 29, 37]. These structure parameters are very close to one another, implying that the Ag nanorod arrays deposited on different thin film substrates are statistically the same. The SERS response of these Ag nanorod substrates were evaluated under identical conditions: A 2 μL droplet of a Raman probe molecule, trans-1, 2- bis (4-pyridyl) ethylene (BPE) with a concentration of 10-5 M, was uniformly dis‐ persed onto the Ag nanorod substrates. The SERS spectra were recorded by the HRC-10HT Raman Analyzer from EnwaveOptronics Inc., with an excitation wavelength of *λ* 0 = 785 nm,

**Figure 6.** Representative SEM images of Ag nanorod arrays on 100 nm underlayer thin films with different materials:

Fig 7(a) shows the representative BPE SERS spectra obtained at *λ* <sup>0</sup> = 785 nm from the Ag nanorod arrays on thin Ag, Al, Si and Ti film underlayers (thickness *d* = 100 nm). Each spec‐ trum is an average of at least 15 different spectra taken at different spots on the substrates. All of them show the three main Raman bands of BPE, Δ*ν* = 1639, 1610, and 1200 cm-1, which

a power of 30 mW and an accumulation time of 10 s.

38 Organic Pollutants - Monitoring, Risk and Treatment

(a) Ag; (b) Al; (c) Ti; (d) Si. All the Figs have the same scale bar.

Fig 7(b) plots the SERS intensity *I* 1200 versus the reflectivity *R* of the underlayer thin films at *λ* 0 = 785 nm. The error bar for the Raman intensity is the standard deviation from 15 or more measurements from multiple sampling spots on the same substrates and the error bar for the reflectivity data is calculated from multiple reflectivity measurements at *λ* 0 = 785 nm. In Fig. 7(b), the SERS intensity and reflectivity follow a linear relationship: when the reflec‐ tivity of the underlayer increases, the SERS enhancement factor increases.

This linear relationship of the underlayer reflectivity and SERS intensity can be explained by a modified Greenler's model developed by Liu *et al*. [34]Greenler's model is proposed through classical electrodynamics to explain the effects of the incident angles and polari‐ zation, and the collecting angle on the Raman scattering from a molecule adsorbed on a planar surface [32]. The modified Greenler's model extended the Greenler's model from a planar surface to Ag nanorod substrates and considered the effect of the underlying sub‐ strate [34]. The main point of the modified Greenler's model is to consider the conditions of both the incident and scattering fields near the molecule absorbed on a nanorod to cal‐ culate the enhancement.

Assuming that relative Raman intensity *η* is the ratio of the total Raman scattering power to incident light power, according to the modified Greenler's model, *η* excited by an unpolar‐ ized light can be explicitly expressed as [34]

$$\begin{split} |\eta| &= \epsilon \mathcal{E}\_{\text{Raman}}^2 > \left| \begin{aligned} &< \mathcal{E}\_{\text{incident}}^2 > & \\ &= \frac{1}{2} \Big{[} \mathbf{1} + \mathcal{R}\_p + n\_2^4 \mathcal{R}\_p^\cdot \cos \delta\_p \cos 2(\eta - \beta) + 2n\_2^2 \mathcal{R}^\cdot \frac{1}{p} \cos(\delta\_p^\cdot + 2\pi\Delta \Big/ \lambda) \sin 2\beta \end{aligned} \right. \\ &+ 2n\_2^2 \mathcal{R}\_p^\cdot \frac{1}{2} \mathcal{R}\_p^\cdot \frac{1}{p} \sin 2\varphi \cos(\delta\_p^\cdot + 2\pi\Delta \Big/ \lambda - \delta\_p) \mathbf{j} (1 + \mathcal{R}\_p + 2\sqrt{\mathcal{R}\_p} \cos \delta\_p) \cos^2(\eta - \beta) \mathbf{j} \\ &+ \left[ 1 + n\_2^4 \mathcal{R}\_s^\cdot + 2n\_2^2 \mathcal{R}\_s^\cdot \frac{1}{s} \cos(\delta\_s^\cdot + 2\pi\Delta \Big/ \lambda) \mathbf{j} \right] \end{split}$$

where *R* p and *R* s are the reflectivity of *p*- and *s*-polarized lights by the Ag nanorod surface, and *R'* p and *R'* <sup>s</sup> are the reflectivity of *p*- and *s*-polarized components by the underlayer thin film; *n* 2 is complex refractive index of Ag, and *n* 2 = 0.03 + 5.242*i* (for *λ* <sup>0</sup> = 785 nm); *φ* is the light incident angle, and *β* is the Ag nanorod tilting angle; *Δ* = *d* (1 + *cos* 2*φ*) / *cosφ*, where *d* is the thickness of Ag nanorod layer; *δ* p, *δ* s, *δ'* p, and *δ'* s are the reflectivity phase shifts of *p*and *s*-polarization E-fields from Ag nanorods and underlayer thin film, defined as

$$\begin{aligned} \delta\_p &= \tan^{-1} \text{Im}(r\_p) / \text{Re}(r\_p) \mathbf{J} \cdot \delta\_s = \tan^{-1} \text{Im}(r\_s) / \text{Re}(r\_s) \mathbf{I} \\ \delta'\_{\,\_p} &= \tan^{-1} \text{Im}(r'\_{\,\_p}) / \text{Re}(r'\_{\,\_p}) \mathbf{J} \cdot \delta'\_s = \tan^{-1} \text{Im}(r'\_{\,\_s}) / \text{Re}(r'\_{\,\_s}) \mathbf{I} \end{aligned}$$

By setting the light incident angle *φ* = 0º, the Ag nanorod tilting angle *β* = 73º, the thickness of Ag layer *d* = 300 nm, the relative Raman intensity *η* as a function of the underlayer reflec‐ tivity *R* at *λ* 0 = 785 nm is calculated and plotted in Fig. 7(c). It shows that the *η* indeed in‐ creases linearly with *R*, which is in very good agreement with our experimental data shown in Fig. 7(b). Therefore, the underlayer reflectivity is one significant parameter to consider for improving the SERS response of multilayer substrates.

strates. Accordingly, in order to further improve the SERS response of the Ag nanorod sub‐ strates, one can further increase the reflectivity of the underlayers through a proper surface

The Detection of Organic Pollutants at Trace Level by Variable Kinds of Silver Film with Novel Morphology

http://dx.doi.org/10.5772/53602

41

Most studies have been focused on controlling the morphology of individual nanostructures to maximize their performance as SERS substrates, without consideration of the effect of the ordered arrangement of the nanostructure(s). Obviously, periodically arranged nanostruc‐ tures might result in an enhancement in the localized electric field due to the resonance with the localized surface Plasmon [40,41] different from that of randomly arranged nanostruc‐ tures, providing a possibility to improve the performance of SERS substrates. Therefore, it is of great interest to investigate the surface-enhanced Raman scattering from those nanostruc‐

The substrates used in the experiment were N-type silicon substrates with an orientation of <100>. Hexagonal lattices (200 μm ×200 μm) of silicon patterns (~ 400nm in diameter) were fabricated on the substrates by electron beam lithography, where the separation distance of the patterns was controlled to be 0 (closely-packed), 50 nm, 100 nm, 200 nm, 300 nm and 400 nm, respectively. These were cleaned in sequence in acetone, ethanol, and de-ionized water baths supersonically, and were mounted on the substrates holder (cooled by liquid nitrogen) in a high vacuum e-beam deposition system (with a background vacuum level better than 2×10-5 Pa). Vertically aligned Ag nanorods were deposited on these substrates by the glanc‐ ing-angle deposition (GLAD) technique described elsewhere [42]. During deposition, the

set ~ 88O off the incoming vapor flux, and the deposition rate was monitored to be ~ 0.5 nm/s

Oblique-view SEM images is shown from Fig.8 of Ag nanorods deposited on the same sili‐ con substrate fabricated by electron beam lithography, with planar areas, and hexagonal lat‐ tice areas (~ 200 μm × 200 μm) of silicon patterns (~ 400nm in diameter) that are separated at 0 nm (closely packed), 50 nm, 100 nm, 200 nm and 300 nm, respectively. Silver nanorods grown on hexagonal lattice areas with other separations are of similar morphology and fea‐ tures. Since the height of silicon micro-patterns etched by electron beam lithography is only ~ 20 nm, there should be also deposition of Ag nanorods in the spaces among the patterns when their separation distance is large enough, e.g., 300 nm, see Fig.8(f). If the separation is not that large, there is normally no deposition of Ag nanorods in these areas because of the shadow effect by the GLAD technique [42]. It is found from the images that Ag nanorods were deposited onto the silicon patterns, forming two-dimensional hexagonal lattices of var‐ ious lattice parameters (or separation distances of patterns). This provides us ideal samples

We tested the performance of the hexagonal lattices of patterns covered by Ag nanorods as SERS substrates using R6G as the probing molecule. The samples are dipped in 1×10-6 M aqueous solution of R6G for 30 minutes and dried by a continuous gentle nitrogen stream. The Raman spectrum of R6G on each SERS substrate was obtained by measuring and aver‐

to investigate the enhancement by the regular arrays to the Raman scattering.

C, rotated at a speed of 2 rpm and its surface normal was

coating such as multilayer dielectric coating39.

tures periodically arranged.

substrate was cooled down to -20 o

using a quartz crystal microbalance.

**2.3. Enhancement of the sensitivity from periodical silver nanorods**

**Figure 7.** a) BPE SERS spectra obtained from Ag nanorod arrays deposited on 100 nm thin Ag, Al, Si and Ti film under‐ layers; (b) the plot of experimental Raman intensity as a function of underlayer reflectivity at λ0 = 785 nm. Different symbol groups represent different kinds of substrates. (c) The plot of the enhanced Raman intensity ratio η as a func‐ tion of underlayer reflectivity calculated by the modified Greenler's model.

Both our experiments and the modified Greenler's model demonstrate that the higher the underlayer reflectivity, the higher the SERS intensity for the Ag nanorod based SERS sub‐ strates. Accordingly, in order to further improve the SERS response of the Ag nanorod sub‐ strates, one can further increase the reflectivity of the underlayers through a proper surface coating such as multilayer dielectric coating39.

### **2.3. Enhancement of the sensitivity from periodical silver nanorods**

By setting the light incident angle *φ* = 0º, the Ag nanorod tilting angle *β* = 73º, the thickness of Ag layer *d* = 300 nm, the relative Raman intensity *η* as a function of the underlayer reflec‐ tivity *R* at *λ* 0 = 785 nm is calculated and plotted in Fig. 7(c). It shows that the *η* indeed in‐ creases linearly with *R*, which is in very good agreement with our experimental data shown in Fig. 7(b). Therefore, the underlayer reflectivity is one significant parameter to consider for

**Figure 7.** a) BPE SERS spectra obtained from Ag nanorod arrays deposited on 100 nm thin Ag, Al, Si and Ti film under‐ layers; (b) the plot of experimental Raman intensity as a function of underlayer reflectivity at λ0 = 785 nm. Different symbol groups represent different kinds of substrates. (c) The plot of the enhanced Raman intensity ratio η as a func‐

Both our experiments and the modified Greenler's model demonstrate that the higher the underlayer reflectivity, the higher the SERS intensity for the Ag nanorod based SERS sub‐

tion of underlayer reflectivity calculated by the modified Greenler's model.

improving the SERS response of multilayer substrates.

40 Organic Pollutants - Monitoring, Risk and Treatment

Most studies have been focused on controlling the morphology of individual nanostructures to maximize their performance as SERS substrates, without consideration of the effect of the ordered arrangement of the nanostructure(s). Obviously, periodically arranged nanostruc‐ tures might result in an enhancement in the localized electric field due to the resonance with the localized surface Plasmon [40,41] different from that of randomly arranged nanostruc‐ tures, providing a possibility to improve the performance of SERS substrates. Therefore, it is of great interest to investigate the surface-enhanced Raman scattering from those nanostruc‐ tures periodically arranged.

The substrates used in the experiment were N-type silicon substrates with an orientation of <100>. Hexagonal lattices (200 μm ×200 μm) of silicon patterns (~ 400nm in diameter) were fabricated on the substrates by electron beam lithography, where the separation distance of the patterns was controlled to be 0 (closely-packed), 50 nm, 100 nm, 200 nm, 300 nm and 400 nm, respectively. These were cleaned in sequence in acetone, ethanol, and de-ionized water baths supersonically, and were mounted on the substrates holder (cooled by liquid nitrogen) in a high vacuum e-beam deposition system (with a background vacuum level better than 2×10-5 Pa). Vertically aligned Ag nanorods were deposited on these substrates by the glanc‐ ing-angle deposition (GLAD) technique described elsewhere [42]. During deposition, the substrate was cooled down to -20 o C, rotated at a speed of 2 rpm and its surface normal was set ~ 88O off the incoming vapor flux, and the deposition rate was monitored to be ~ 0.5 nm/s using a quartz crystal microbalance.

Oblique-view SEM images is shown from Fig.8 of Ag nanorods deposited on the same sili‐ con substrate fabricated by electron beam lithography, with planar areas, and hexagonal lat‐ tice areas (~ 200 μm × 200 μm) of silicon patterns (~ 400nm in diameter) that are separated at 0 nm (closely packed), 50 nm, 100 nm, 200 nm and 300 nm, respectively. Silver nanorods grown on hexagonal lattice areas with other separations are of similar morphology and fea‐ tures. Since the height of silicon micro-patterns etched by electron beam lithography is only ~ 20 nm, there should be also deposition of Ag nanorods in the spaces among the patterns when their separation distance is large enough, e.g., 300 nm, see Fig.8(f). If the separation is not that large, there is normally no deposition of Ag nanorods in these areas because of the shadow effect by the GLAD technique [42]. It is found from the images that Ag nanorods were deposited onto the silicon patterns, forming two-dimensional hexagonal lattices of var‐ ious lattice parameters (or separation distances of patterns). This provides us ideal samples to investigate the enhancement by the regular arrays to the Raman scattering.

We tested the performance of the hexagonal lattices of patterns covered by Ag nanorods as SERS substrates using R6G as the probing molecule. The samples are dipped in 1×10-6 M aqueous solution of R6G for 30 minutes and dried by a continuous gentle nitrogen stream. The Raman spectrum of R6G on each SERS substrate was obtained by measuring and aver‐ aging spectra from six different areas of the same hexagonal lattice, by a Reinshaw 100 Ram‐ an spectrometer using a 633 nm He-Ne laser as the excitation source. During measurements the spot size of the laser beam was defocused to ~ 10 μm in diameter, and the laser power was decreased to 0.47 mW to avoid any damage to the R6G molecules, signal accumunition time of 10 second per 600 cm-1, 10x objective and NA 0.25. To make a clear comparison of the performance of these SERS substrates, the Raman spectrum of R6G from the unpatterned areas was also measured.

**Figure 9.** a) Raman spectra of R6G (concentration of 10-6 M) using aligned Ag nanorods deposited on unpatterned silicon substrates and on hexagonal lattices of silicon patterns separated by 50, 200 and 300 nm, respectively (To make each spectrum clear, the four spectrum is separated by adding value 0, 6500, 28000 and 14000, respectively), as SERS substrates; and (b) comparison of the Raman spectrum of R6G obtained from six different regions of the hexago‐

The Detection of Organic Pollutants at Trace Level by Variable Kinds of Silver Film with Novel Morphology

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43

Raman spectra is shown from Fig.9(a) of R6G molecules measured from different areas of the substrate, i.e., the unpatterned area (as the reference), and the hexagonal lattice areas of silicon patterns that are separated at 50 nm, 200 nm and 300nm. We see that each spectrum shows clearly the Raman features of the R6G molecules, and that the signal intensity of the Raman spectrum is very dependent on the substrate (or the separation distance of the pat‐ terns). For example, the signal intensity is the highest when the separation distance is 200 nm, and is the lowest in the unpatterned area. Difference in the Raman intensity between the unpatterned area and the hexagonal lattice areas is noticed. Fig.9(b) compares Raman spectra measured from six different areas of the closely-packed lattice, from which one sees that these spectra match quite well. This suggests a good homogeneity of the Raman signals

nal lattice that are separated at 50 nm of aligned Ag nanorods.

from these SERS substrates.

**Figure 8.** Olique-view SEM images of vertically aligned Ag nanorods deposited on (a) unpatterned silicon; and on hex‐ agonal lattices of silicon patterns that are separated by (b) 0 nm (closely-packed); (c) 50 nm; (d) 100 nm; (e) 200 nm; and (f) 300 nm, respectively.

aging spectra from six different areas of the same hexagonal lattice, by a Reinshaw 100 Ram‐ an spectrometer using a 633 nm He-Ne laser as the excitation source. During measurements the spot size of the laser beam was defocused to ~ 10 μm in diameter, and the laser power was decreased to 0.47 mW to avoid any damage to the R6G molecules, signal accumunition time of 10 second per 600 cm-1, 10x objective and NA 0.25. To make a clear comparison of the performance of these SERS substrates, the Raman spectrum of R6G from the unpatterned

**Figure 8.** Olique-view SEM images of vertically aligned Ag nanorods deposited on (a) unpatterned silicon; and on hex‐ agonal lattices of silicon patterns that are separated by (b) 0 nm (closely-packed); (c) 50 nm; (d) 100 nm; (e) 200 nm;

areas was also measured.

42 Organic Pollutants - Monitoring, Risk and Treatment

and (f) 300 nm, respectively.

**Figure 9.** a) Raman spectra of R6G (concentration of 10-6 M) using aligned Ag nanorods deposited on unpatterned silicon substrates and on hexagonal lattices of silicon patterns separated by 50, 200 and 300 nm, respectively (To make each spectrum clear, the four spectrum is separated by adding value 0, 6500, 28000 and 14000, respectively), as SERS substrates; and (b) comparison of the Raman spectrum of R6G obtained from six different regions of the hexago‐ nal lattice that are separated at 50 nm of aligned Ag nanorods.

Raman spectra is shown from Fig.9(a) of R6G molecules measured from different areas of the substrate, i.e., the unpatterned area (as the reference), and the hexagonal lattice areas of silicon patterns that are separated at 50 nm, 200 nm and 300nm. We see that each spectrum shows clearly the Raman features of the R6G molecules, and that the signal intensity of the Raman spectrum is very dependent on the substrate (or the separation distance of the pat‐ terns). For example, the signal intensity is the highest when the separation distance is 200 nm, and is the lowest in the unpatterned area. Difference in the Raman intensity between the unpatterned area and the hexagonal lattice areas is noticed. Fig.9(b) compares Raman spectra measured from six different areas of the closely-packed lattice, from which one sees that these spectra match quite well. This suggests a good homogeneity of the Raman signals from these SERS substrates.

when the patterns are separated at ~ 200 nm, the signals are further enhanced by ~5 times compared with the unpatterned substrates. So the performance of silver nanostructure as

The Detection of Organic Pollutants at Trace Level by Variable Kinds of Silver Film with Novel Morphology

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With the highly sensitive SERS substrates described before, one can detect trace amount or‐

SERS is extremely sensitive in water solutions, for water does not have any Raman peaks. When detecting organic pollutants in nonaqueous systems, we use volatile organic solvents, such as acetone, to dilute pollutants. As the organic solvent shows high Raman background,

The powders of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl used in this study were commercial‐ ly available fromthe AccuStandard Company. Since there is no Raman data of 2, 3, 3′, 4, 4′ pentachlorinated biphenyl reported, we first measured its Raman spectrum and that of acetone, for comparison, see Fig 11(a). To clearly show most characteristic peaks of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl, the Raman spectrum was plotted in two regions of 300 to 1000 cm-1 and 1000 to 1700 cm-1 respectively, see Figs 11(b) and 14(c). From the Figs one sees that the strongest peaks are located at 342, 395, 436, 465, 495, 507, 517, 598, 679, 731, 833, 891, 1032, 1136, 1179, 1254, 1294, 1573, and 1591 cm-1, respectively; while for acetone the charac‐ teristic peaks are at 530, 786, 1065, 1220, 1428 and 1709 cm-1, respectively. It is suggested that 2, 3, 3′, 4, 4′-pentachlorinated biphenyl is distinguishable from acetone and that acetone can be used as the solvent for the SERS measurements, as 2, 3, 3′, 4, 4′-pentachlorinated biphen‐

Because the SERS sensitivity is also dependent on the sample treatment, we employed in this study a very simple method to prepare SERS samples, i.e. dropping a small volume (~ 0.5 uL) of solutions of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl in acetone on Ag nanorods using a top single channel pipettor and then blowing away the acetone with a continuous, gentle nitrogen blow. Fig 12(a) shows the Raman spectra of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl dissolved in acetone at concentrations of 10-4 to 10-10mol/L, respectively. The accu‐ mulation time of each Raman spectrum was 50 seconds and we used only 1% laser power to avoid changing of pentachlorinated biphenyl. When the small volume (~ 0.5 uL) of solutions of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl was dropped on Ag nanorods, it became a circular spot with diameter of about 4 mm. The Raman spectrum was accumulated from a 2 um di‐ ameter circular area on the substrates. Therefore, for the solution at concentration of 10-10mol/L, only about ten 2, 3, 3′, 4, 4′-pentachlorinated biphenyl molecules (2\*10-23mol) would be accumulated in SERS; if that was at concentration of 10-8mol/L, about 1000 mole‐

we need to make the solvent volatilizated completely before SERS measurement.

SERS substrate is improved through periodical silver nanorods. [43]

**3. Detection of PCBs at trace amount by SERS**

ganic molecules by the SERS method.

yl is not soluble in water.

cules (2\*10-21mol) would be accumulated and so on.

**Figure 10.** The intensity of the two characteristic peaks of R6G on hexagonal lattices of Ag nanorods: (a) 612 cm-1 and (b) 1508 cm-1.

To gain a better understating of the latticing effect on the Raman scattering, we plot the in‐ tensity of the two characteristic peaks of R6G molecules, i.e. at ~ 612 cm-1 and ~ 1508 cm-1, as a function of the separation distance of the lattice patterns, by comparing with the unpat‐ terned Ag film (the two red dash line represent the sum and difference between the average and uncertainty of signal strength). Fig.10(a) and 10(b) shows respectively the intensity of the two peaks versus the separation distance of the lattice patterns. It is seen from the fig‐ ures that for the two peaks, they demonstrated very similar dependence behavior on the separation distance. In comparison with signals from the unpartterned area, we notice that when the patterns are separated at ~ 200 nm, the signals are further enhanced by ~5 times compared with the unpatterned substrates. So the performance of silver nanostructure as SERS substrate is improved through periodical silver nanorods. [43]
