A General Method for Large-Scale Fabrication of Semiconducting

Apr 11, 2017 - Here we report a general and versatile method, based on ion irradiation and vacuum annealing, for fabricating large-scale reduced ...
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A General Method for Large-Scale Fabrication of Semiconducting Oxides with High SERS Sensitivity Xudong Zheng, Feng Ren, Shunping Zhang, Xiaolei Zhang, Hengyi Wu, Xingang Zhang, Zhuo Xing, Wenjing Qin, Yong Liu, and Changzhong Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03839 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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A General Method for Large-Scale Fabrication of Semiconducting Oxides with High SERS Sensitivity Xudong Zheng,† Feng Ren,*,†,

‡

Shunping Zhang,‡ Xiaolei Zhang,† Hengyi Wu,†

Xingang Zhang,† Zhuo Xing,† Wenjing Qin,† Yong Liu,‡ Changzhong Jiang*,†,‡ †School of Physics and Technology, Center for Ion Beam Application, Center for Electron Microscopy and Hubei Nuclear Solid Physics Key Laboratory, Wuhan University, Wuhan 430072, China ‡

School of Physics and Technology, MOE Key Laboratory of Artificial Micro- and

Nano-structures, Wuhan University, Wuhan 430072, China KEYWORDS: ion irradiation, semiconducting oxide, oxygen vacancy, charge transfer, surface-enhanced Raman scattering

ABSTRACT: Surface enhanced Raman spectroscopy (SERS) is a versatile and powerful spectroscopic technique for substance analysis and detection. So far, the highest detection sensitivities have been realized on noble nanostructure substrates, which, however, are costly, unstable and non-biocompatible. While semiconductor substrates could in principle be used, existing realizations have either resulted in substrates with low sensitivities or used methods that have poor technical control. Here we report a general and versatile method, based on ion irradiation and vacuum annealing, for fabricating large-scale reduced semiconducting oxide SERS substrates with high sensitivities. The SERS enhancement mainly stems from oxygen vacancy-associated electronic states created by the ion irradiation of sample; these 1

states enhance the charge transfer (CT) mechanism between the oxide substrate and the adsorbed molecules and thus significantly magnify SERS signals. The improved carrier mobility by vacuum annealing and the introduction of impurity energy levels and nanostructures enhances further the CT efficiency. A detection limit as low as 10-8 M was achieved; this is the highest sensitivity among the reported semiconductors, and even compares to noble metals without the aid of ‘hot spots’. The method is general - we demonstrate it on WO 3 , ZnO and TiO 2 substrates using Ar+ and N+ ion beam irradiation - and broadly applicable to produce noble-metal-free SERS substrates with high sensitivities.

1. INTRODUCTION Surface-enhanced Raman scattering (SERS) is a rapid and sensitive spectroscopic technique that has attracted tremendous attention due to its potential for probing a wide range of sensing processes and mechanisms of molecules in the fields of analytical chemistry, catalysis, and biochemical detection.1-4 Noble metals have been widely investigated5-8 for SERS because of their efficient enhancement of Raman signals due to the strong electromagnetic enhancement resulting from surface plasmon resonance.9-11 However, the shortcomings of noble metal SERS substrates, such as high cost, poor stability and biocompatibility, largely limit their practical applications. Semiconductor

materials

could

in

principle

address

these

shortcomings.

Semiconductor substrates have been proven to be SERS-active12-16 and could

2

potentially have considerable advantages, over noble metals, with respect to opening up for analyses of the electrical properties of the semiconductor/metal interface,17,18 inorganic species and metal ions.19,20 However, a serious problem of these semiconductor substrates is their relatively low SERS sensitivities. The challenge is thus how to develop SERS-active semiconductor substrates with high SERS sensitivity. It is believed that the SERS effect of semiconductor materials is due to the chemical enhancement mechanism based on the charge-transfer (CT).21 When excited by laser light, electrons move from the adsorbed molecules to the semiconductor material or vice versa. This CT process leads to an enormous magnification of the molecular polarizability, which results in Raman scattering enhancement. But the CT mechanism depends on whether the band-gap structures of the semiconductor and the molecular orbitals are matched in energy to each other; this is usually not the case, which is the main reason why the SERS sensitivity of semiconductors is relatively low. Tuning the band-gap structure of semiconductors may provide a way to solve this problem. WO 3 , TiO 2 and ZnO have been extensively studied, because they possess many advantages, such as high stability, low cost and excellent optical propertities. Furthermore, oxygen vacancies in WO 3 , TiO 2 and ZnO can act as shallow donor levels and significantly improve their conductivity and optical properties. Thus, they have potential to achieve the goal of high SERS sensitivity comparable to that of noble metals. Introducing of oxygen vacancy is one efficient

3

strategy in semiconducting oxides.22,23 However, the so far reported methods, such as hydrothermal approach,22 thermal treatment in a reduction atmosphere24 and anodization technique,25 are difficult to use to control the concentration of vacancies. For example, for thermal treatment in a reduction atmosphere, the key problem is that the shallow distribution of oxygen vacancies leads easily to the oxidation of oxygen vacancies by oxygen from air. Here we report a general and versatile method, based on ion irradiation and vacuum annealing, for the fabrication of large-scale, low-cost, highly stable and sensitive semiconductor SERS substrates (WO 3-x , ZnO 1-x , TiO 2-x ) (Figure 1). The method based on ion beam technique is a widely used and powerful method for semiconductor doping and modification (e.g. in the integrated circuit industry).26-30 Herein self-doping, doping and nanostructures are achieved simultaneously and controlled by tuning the irradiation parameters and annealing conditions, which can boost the charge transfer between the semiconductor and molecule. Using our approach, we demonstrate the highest SERS-sensitivities among the reported semiconductor SERS-active substrates - a detection limit as a low as 10-8 M comparing to the reported values ranging from 10-3 to 10-7 M.

4

Figure 1. Fabrication process of the reduced WO 3-x film and SERS measurement of Rhodamine 6G (R6G) molecules.

2. EXPERIMENTAL SECTION 2.1. Fabrication of the Reduced WO 3-x and TiO 2-x . Pristine WO 3 thin films were deposited at 400 °C on SiO 2 slides using ultrahigh vacuum magnetron sputtering system (ULVAC, ACS-400-C4). The thickness of the film was about 70 nm. 90-nm-thick ZnO films were deposited at 200 °C on silica slices using magnetron sputtering system. TiO 2 (001) rutile single crystal wafers were purchased. Ar+ ion irradiation of the WO 3 thin films and TiO 2 (001) single crystal were performed at room temperature using a 200 kV ion implanter. The pristine WO 3 samples were irradiated by 190 keV Ar+ ions to fluences of 1×1015, 5×1015, 1×1016, 3×1016, 6×1016 and 1×1017 ions/cm2, and by 80 or 130 keV Ar+ ions to the fluence of 1×1017 ions/cm2, respectively. The ZnO films were irradiated by Ar+ ions at 190 keV to the fluence of 6×1016 ions/cm2. Single crystal TiO 2 slides were irradiated by 50 keV Ar+ ions to the fluence of 1×1017 ions/cm2. The ion flux was 1~2 μA/cm2. To prove the universality of the reported method, N+ irradiation of TiO 2 single crystal was carried out on an ion implanter equipped with a Kaufman ion source. The acceleration voltage was 45 kV and the fluence was 1×1017 ions/cm2. The acceleration current was about 1 mA. The irradiated WO 3 and TiO 2 were annealed in vacuum at 500 °C for 1 h in a tube furnace. The irradiated ZnO annealed in vacuum at 400 °C for one hour. For vacuum

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annealing, the pressure was kept at 2×10-4 Pa. The 3×1016 and 6×1016 Ar+ ions/cm2 (Ar+ ions at 190 keV) irradiated WO 3 films were annealed at 500 °C for 1 h in flowing oxygen gas and the N+ irradiated and vacuum-annealed TiO 2 single crystal were annealed at 400 °C for 1 h in flowing oxygen gas. 2.2. SERS Measurements. To study the SERS effect from the irradiated samples, R6G as the probe molecule was dissolved in deionized water. The substrates were immersed in the prepared R6G solution for five hours. Raman spectra were collected on a confocal Raman spectrometers (RenishawinVia, Renishaw) using 532 nm laser. The laser power was maintained at 0.25 mW (0.5 % power), and data acquisition time was kept at 10 s. The laser spot focused through a 100× objective lens has a diameter of 1 μm. 2.3. Material Characterization and Measurement. The morphologies and microstructures were characterized using a field- emission scanning electron microscope (FE-SEM) (Hitachi S-4800) and a JEOL 2010FEF (UHR) microscope operating at 200 kV. Optical absorption spectra were obtained using a UV-vis-NIR dual-beam spectrophotometer (Varian Cary 5000) with wavelengths from 800 to 200 nm. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific ESCALAB 250 Xi system. Al kα (1486.6 eV) was used as the radiation source. The binding energy of the obtained XPS spectra was corrected using the C 1s level at 284.8 eV. Structural analysis was performed in a Rigaku Ultima IV Advanced X-ray diffractometer by grazing incidence X-ray diffraction (GIXRD). The dc

6

electrical resistivity measurement was done by a standard two-probe method. Ohmic contacts were prepared by coating indium layer. The carrier mobility and concentration were measured on a Lakeshore 7704 Hall measurement system. The magnetization was measured at 300 K by using a vibrating-sample magnetometer (VSM) on a physical property measurement system (PPMS-9, Quantum Design) with the applied field varying from -3000 to 3000 Oe. 3. RESULTS AND DISCUSSION 3.1. Ion-Irradiation-Induced Formation of Oxygen Vacancies in WO 3 Films Oxygen vacancies in WO 3 that act as donors, and thus improve the conductivity and narrow the band gap,22,31 have potential for enhancing the CT process. In our proof-of-principle demonstration we first irradiated 70-nm-thick WO 3 films with a flat surface deposited on silica slices (Supporting Information Figure S1) with 190 keV Ar+ ions at room temperature to fluences from 1×1015 to 1×1017 ions/cm2. Pristine WO 3 crystal is composed of slabs of corner-sharing distorted WO 6 octahedra. According to SRIM simulation based on the Monte Carlo method,32 energetic-ion collision leads to the formation of displaced atoms (O and W). The displaced atoms stay in the interstitial positions and partial displaced atoms near the sample surface are sputtered out of the material, leaving behind the corresponding vacancies. As a result, WO 3 is reduced to WO 3-x . The depth profiles of the irradiation-induced vacancies in WO 3 simulated by the SRIM program indicate that oxygen vacancy concentration is greatly higher than tungsten vacancy concentration, and there is a gradient distribution

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from external to internal in the cross section of the 70-nm-thick film (Supporting Information Figure S2a). Since the projected range of 190 keV Ar+ ions is larger than the thickness of the WO 3 films and most of Ar+ ions penetrate the films (Figure S2b). Both oxygen vancancies and other unfavorable damages (W interstitials and vacancies, and amorphization) were formed during ion irradiation. To recover the damages, the irradiated samples were annealed in vacuum at 500 °C for one hour.

8

Figure 2. SEM images of the irradiated WO 3 by (a) 3×1016, (c) 6×1016 and (e) 1×1017 Ar+ ions/cm2 and (b, d, and f) the SEM images of the corresponding annealed samples in vacuum. HRTEM images of (g) the pristine WO 3 and (h) the 3×1016 Ar+ ions/cm2 irradiated and vacuum-annealed WO 3 .

The scanning electron microscopy (SEM) images reveal changes in surface morphology upon irradiation of the WO 3 sample (Figure 2a, c and e) that we attribute to aggregation of a large number of vacancies. While the surface morphology did not change distinctly with fluences lower than 3×1016 ions/cm2 (Figure 2a), we observed the formation of nanopores with diameters between 50 and 100 nm at 6×1016 ions/cm2 (Figure 2c). When the fluence further increases to 1×1017 ions/cm2, the diameter of the nanopores increased to around 200 nm (Figure 2e). According to calculations with the SRIM program, the projected range of Ar+ ions with the energy of 190 keV is larger than the film thickness (70 nm), which means that most of incident ions penetrate the film. However, the maximum density of the Ar+ ions in the film reaches 6% (~1020 atoms/cm2) for high-fluence (1×1017 ions/cm2) irradiation. Moreover, the concentration of vacancy in the irradiated sample is also high (Supporting Information Figure S2a). The aggregation of both vacancies and Ar atoms lead to the formation of large surface pores.Post-irradiation vacuum annealing did not significantly change the surface morphology (Figure 2b, d and f). Figure 2g, h shows high-resolution transmission electron microscopy (HRTEM) images of the pristine WO 3 and the

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3×1016 Ar+ ions/cm2 irradiated and vacuum-annealed WO 3 . The HRTEM image reveals continuous lattice fringes in the pristine WO 3 with an interplanar spacing of 0.38 nm; these fringes belong to the {002} atomic planes of monoclinic WO 3 (JCPDS Card No. 83-0951). However, in the irradiated and vacuum-annealed WO 3 , we observed numerous dislocations and lattice disorder that suggest the existence of a large number of oxygen vacancy defects.24,33

Figure 3. Raman spectra of the pristine WO 3 and the irradiated WO 3 to different fluences (a) before and (b) after annealed in vacuum and in O 2 at 500 °C.

A large number of oxygen vacancies induced by the irradiation are also shown in our Raman measurements. The bands at 270, 330, 715, and 807 cm-1 observed in the pristine monoclinic WO 3 34 disappear after ion irradiation due to the extensive structural damage induced by the irradiation (Figure 3a). After vacuum annealing, a broad and weak band in the 600-900 cm-1 region appears in the irradiated samples (Figure 3b), which reveals an improvement in crystallinity and a partial recovery from

10

the damage. But the intensity of the bands decreases gradually when the fluence is increased, which suggests that the damage concentration increases with increasing fluence. We also annealed a WO 3 sample irradiated with 3×1016 Ar+ ions/cm2 in oxygen atmosphere, which recovered the Raman bands of monoclinic WO 3 ; the intensities of these bands were stronger than those of the irradiated and vacuum-annealed WO 3 (Figure 3b), which reveals the existence of lattice damages due to the deficiency of O atoms in the latter.24 Our optical images of sample color changes from light grey to dark blue (Supporting Information Figure S3a) and an increase of the optical absorption in visible-light region (Figure S3b, c) also confirmed that irradiation creates a large number of oxygen vacancies. To further understand the influence of the oxygen vacancies on the WO 3 film we measured the conductivity (Supporting Information Table S1). The conductivity significantly increases up to 106 times larger in the irradiated WO 3 compared to the pristine WO 3 , which we attribute to the formation of shallow donors related to oxygen vacancies.35 Donor defects are generated via the formation of oxygen vacancy from the nuclear collisions: Kinetic energy + O2- (lattice)

1/2O 2 (interstitial or out of system) + Vo∙∙ + 2e-. (1)

Each oxygen vacancy can generate two free electrons, which improves the carrier concentration. Furthermore, we found vacuum annealing further increases the conductivities of all the irradiated samples, which indicates that most of the oxygen vacancies remain in the samples.

11

Figure 4. XPS spectra of W 4f core levels for the irradiated WO 3 to different fluences (a, c, e, g) before and (b, d, f, h) after thermal treatment in vacuum. The black curves correspond to the experimental data. Each black curve is deconvoluted into three pairs 12

of peaks corresponding to W6+ (blue curves), W5+ (orange curves), and W0 (magenta curves). The red curves are the summation of the deconvoluted peaks.

Table 1. Atomic percentages of W6+, W5+ and W0 in the surface of the irradiated WO 3 before and after annealing in vacuum. Fluence (ions/cm2) 1×1016 3×1016 6×1016 1×1017 Pristine

Before annealing W6+ W5+ W0 88.5% 11.5% 73.3% 19.3% 7.4% 56.4% 21.4% 22.2% 51.9% 19.1% 29% 100%

After annealing W W5+ W0 83.2% 16.8% 79.1% 20.9% 62.3% 22.6% 15.1% 56.5% 21.7% 21.8% 93.5% 6.5% 6+

We also performed surface analysis by X-ray photoelectron spectroscopy (XPS) to understand the structural changes induced by irradiation. Figure 4 shows XPS spectra of W 4f core levels for the irradiated WO 3 before and after thermal annealing in vacuum. For the pristine WO 3 , two sharp peaks were observed at 35.5 and 37.7 eV (Supporting Information Figure S4), which represents the emissions from W 4f 5/2 and 4f 7/2 core levels of the W atoms in the 6+ oxidation state. 36 After ion irradiation, the peaks of W 4f become slightly broader and new peaks appear at the lower binding energy region, which represent W5+ (centered at 37 and 34.3 eV)36 and metallic tungsten W0 (31.2, 31.6 and 33.4 eV).37 According to the fitting analysis of XPS spectra of W 4f core levels, the corresponding atomic percentage of W6+, W5+ and W0 are estimated (Table 1). Both the atomic percentages of W5+ to W6+ and W0 to W6+ increase with increasing fluence. The appearance of W5+ after ion irradiation 13

demonstrates the formation of oxygen vacancies.22,24,35 The electrons located on the oxygen vacancy states are driven away and transferred to W6+, resulting in the formation of W5+ defects.31 The formed interstitial W atoms as the unfavorable damage can aggregate to form metal W clusters. After post-irradiation annealing in vacuum, the percentages of W6+ were increase while that of W0 is decreased. Annealing stimulated the diffusion of interstitial W atoms (W0) into the W vacancy and W0 atoms are oxidized into W6+, which suggests a reduction of the damage density. The percentage of W5+ did not change distinctly after annealing in vacuum, which suggests most of the oxygen vacancies remained. Furthermore, the oxygen deficiency (x) in the surface of the induced WO 3-x by ion irradiation to the fluences of 1×1016, 3×1016, 6×1016 and 1×1017 ions/cm2 and vacuum annealing are estimated to be 0.08, 0.10, 0.13 and 0.14, respectively, which increases with increasing irradiation fluence. The results suggest that ion irradiation is a powerful method for tailoring the oxygen-deficiency and it also can achieve heavy vacancy-doping.

14

Figure 5. (a) The GIXRD pattern (black line) of the pristine WO 3 film and a reference pattern (red line) of WO 3 (JCPDS No. 43-1035). (b) The GIXRD pattern (black line) of the 1×1016 Ar+ ions/cm2 irradiated and vacuum-annealed WO 3 film and a reference pattern (red line) of WO 3 (JCPDS No. 36-0103).

To provide the surface (average) composition, the pristine WO 3 and the irradiated and vacuum-annealed WO 3 were analyzed by GIXRD (see Figure 5). Two sharp reflections at 23.2° and 33.3° were observed from the pristine WO 3 , which can be assigned to (002) and (022) facets of monoclinic WO 3 phase (JCPDS No. 43-1035). The dominant crystal facet was (002), and the deposited film grew along in the [001] orientation. After ion irradiation and annealing in vacuum, the dominant diffraction peak appeared at 23.5°, which can be from (010) facet of the monoclinic W 24 O 68 (JCPDS No. 36-0103), a stable phase38. Besides, other reflections match with reference reflections of the monoclinic W 24 O 68 . The wide diffraction peaks of the spectrum indicate that small amount of WO 3 still exist. The formation of monoclinic W 24 O 68 (WO 2.83 ) was due to the formation of oxygen vacancies by ion irradiation. Furthermore, we have demonstrated that ion irradiation can tune the concentration of oxygen vacancies, which suggest that substoichiometric structures of WO 3-x can be formed. Several substoichiometric structures of WO 3-x , where x changes from 2.625 to 2.92, have been experimentally observed39. 3.2. Significantly Enhanced SERS Sensitivity

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Figure 6. (a) Raman spectra of R6G (1×10-5 M) on the irradiated WO 3 to different fluences. The characteristic peaks at 612, 773, 1360 and 1650 cm-1 belong to the different vibration modes of R6G molecules. (b) Raman spectra of R6G (1×10-6 M) on the irradiated and subsequently annealed WO 3 in vacuum. (c) Raman spectra of R6G collected from the 1×1017 Ar+ ions/cm2 irradiated and subsequently annealed WO 3 in vacuum under different R6G concentrations. (d) Raman spectra of R6G (1×10-6 M) collected from the 3×1016 Ar+ ions/cm2 irradiated WO 3 annealed in 16

vacuum and in oxygen. (e) Valence band XPS spectra of the pristine WO 3 , the 3×1016 ions/cm2 irradiated WO 3 annealed in O 2 and the 3×1016, 6×1016 and 1×1017 Ar+ ions/cm2 irradiated WO 3 annealed in vacuum. (f) Energy-level diagram of R6G molecules on oxygen-deficient WO 3-x .

To investigate the enhancement of the Raman signal of the irradiated WO 3 films, we used dye Rhodamine 6G (R6G) as a SERS probe molecule and excited the film and R6G system with the laser light with a wavelength of 532 nm. The detection limit of the irradiated sample was 10-5 M as we observed clear SERS signals from R6G (1×10-5 M) (Figure 6a); while no signal from R6G (1×10-6 M) was detected (Supporting Information Figure S5a). In contrast, the pristine WO 3 had only a detection limit of 10-1 M (Figure S5b). Thus, ion irradiation yielded an enhancement of 104 for the SERS sensitivity. We attribute the observed large enhancement of the SERS sensitivity mainly to the existence of numerous oxygen vacancies, rather than any increase in the surface roughness. This is because we also observe the large SERS enhancement also in the sample irradiated at a low fluence (< 3×1016 ions/cm2), in which case the surface roughness does not change distinctly. Vacuum annealing and increasing the irradiation fluence further enhance the SERS sensitivity (Figure 6b). Vacuum-annealing lowered the detection limit to 1×10-6 M. Increase of irradiation fluence yielded the strongest SERS signals in the samples irradiated to 6×1016 and 1×1017 Ar+ ions/cm2 and vacuum-annealed. For the 1×1017

17

Ar+ ions/cm2 irradiated and vacuum-annealed WO 3 sample, Figure 6c shows the Raman spectra of Rhodamine 6G (R6G) with different concentrations of 1×10-6, 1×10-7, 5×10-8 M. Comparing these Raman spectra, we can conclude the detection limit of 10-7 M is achieved, which is equal to the lowest reported value for reduced WO 2.7 .22 The enhancement factor (EF) was estimated to be about 1.1×104 (see details in the Supporting Information). The origin of the observed enhancement of the Raman after vacuum annealing can be explained as follows. The irradiation fluence is a key parameter to tune the vacancy concentration and damage level. With increasing fluence, more vacancies are produced and the carrier densities reach very high values, ~1022 cm-3 (Supporting Information Table S2), which improves the SERS activity. However, lattice damage, such as interstitials, amorphization, is also aggravated according to our XPS (Figure 4) and conductivity analyses (Supporting Information Table S1). As a result, more grain boundaries and imperfect crystal is produced. They act as scattering centers that diminish the carrier mobility40 - this is supported by our Hall measurements (Supporting Information Table S2) - and block the charge-transfer between WO 3-x and R6G, leading to the decrease of SERS activity. As shown in Figure 6b, no further improvement in SERS activity from 6×1016 to 1×1017 ions/cm2 is also due to heavier lattice damage. After vacuum annealing, the lattice damage was recovered to some extent and the transportation of electron is increased. The combination of the improvement of the carrier mobility and the remain of most oxygen vacancies during vacuum annealing contributes to a more efficient charge

18

transfer processes. The disappearance of SERS signals (Figure 6d) after oxygen annealing, consistent with the large decrease of the conductivity (Supporting Information Table S1), further supports the great contribution of the oxygen vacancy to the SERS enhancement. We also investigated how energy of the ions used in the irradiation influences the Raman activity under same fluence. As shown in the Raman spectra (Figure 6b), both the 6×1016 and 1×1017 Ar+ ions/cm2 irradiated and vacuum-annealed WO 3 show larger Raman enhancement and there is no obvious difference in their SERS intensities. Thus, here we fixed the fluence to 1×1017 ions/cm2 for further study. We found that the SERS intensity decreases with the decrease of ion energy (Supporting Information Figure S7a), which is due to the production of heavier damage as supported by the SRIM simulation (Figure S7b) and the decrease of the conductivity of the corresponding irradiated WO 3 (Supporting Information Table S3). The study of the influence of irradiation energy on SERS activity indicates that there is a balance between producing more vacancies using lower ion energy and the irradiation induced structure damage which blocks the transfer of charges. Therefore, chosing situable ion energy is necessary. To further understand the CT mechanism, we used XPS to study the surface electronic states associated with oxygen vacancies in irradiated WO 3 (Figure 6e). Compared to the pristine WO 3 , the irradiated and vacuum-annealed WO 3 films show clear and well-defined peaks near the Fermi level (the zero point of the binding

19

energy) in the valence band XPS spectra. Their appearance indicates the introduction of new oxygen vacancy-associated energy levels (Vo) in the band structure of the irradiated WO 3 .41,42 Under 532 nm laser excitation, several thermodynamically feasible resonances can occur, including exciton resonance of the introduced defect states, resonance of R6G molecule and charge transfer resonances from WO 3-x and R6G (Figure 6f).22 These additional resonances related to the defect states from oxygen vacancy will lead to a dramatic increase in the Raman scattering cross-section. Oxygen annealing caused an obvious reduction of the peak intensity in the irradiated WO 3 , which indicates that the decrease of the corresponding SERS sensitivity (Figure 6d) is due to the absence of the additional resonances related to oxygen vacancy because of the decrease of oxygen vacancy concentration. However, recently it has been discovered that nanostructured semiconducting oxides with oxygen vacancies after surface oxidation shows SERS activity due to the strong electromagnetic enhancement resulting from localized surface plasmon resonance (LSPR).23 In our work, the as-prepared WO 3-x film after surface oxidation can not exhibit similar activity and thus SERS from the film is mainly due to the CT mechanism. 3.3. Generality and Versatility of Ion Irradiation Method According to the above results, the outstanding SERS performance of the irradiated and vacuum-annealed WO 3 is attributed to two factors: (a) the formation of oxygen vacancy-associated electronic states; (b) better crystallinity required to obtain the high electron mobility. We demonstrated that ion irradiation is a powerful method for

20

tailoring the high concentration of vacancies and that vacuum annealing can improve the lattice structure and carrier mobility (Figure 7). Therefore, ion irradiation combined with vacuum annealing has the potential to be a general and versatile strategy to fabricate reduced semiconducting oxide SERS substrates with high sensitivity.

Figure 7. WO 3 lattice structure illustration before and after ion irradiation and post-irradiation annealing in vacuum.

To demonstrate the generality and versatility of this method, we have also successfully applied it to enhance the sensitivity of TiO 2 substrates. The mechanism for the SERS enhancement for TiO 2 is similar to that of the WO 3 . Irradiation of the sample by 50 keV, 1×1017 Ar+ ions/cm2 revealed clear evidence of the formation of oxygen vacancies. Specifically, in response to the irradiation of the pristine sample we observed color change of the sample from light yellow to dark (Supporting Information Figure S8a) and a dramatic decrease of the electrical 21

resistivity (Supporting Information Table S4) - all of these results point to the formation of shallow donor states associated with oxygen vacancies in the irradiated TiO 2 sample. We also used Raman spectra to characterize the crystal quality of the irradiated samples. The Raman spectra presented in Figure S8b is measured without R6G molecular. A reduction in the Raman intensity was observed in the N+ or Ar+ irradiated and annealed TiO 2 , indicating the producing of O vacancies and lattice damage. This is further supported by the observation of ferromagnetic hysteresis (Figure S8c).43,44 The surface morphology did not change much (Supporting Information Figure S9).

Figure 8. Raman spectra of R6G with various concentrations adsorbed on (a) the 50 keV, 1×1017 Ar+ ions/cm2 irradiated and vacuum-annealed TiO 2 , and (b) the 1×1017 22

N+ ions/cm2 irradiated and vacuum-annealed TiO 2 . (c) The SEM image of the TiO 2 nanorods induced by N+ ion irradiation followed by annealing in vacuum. (d) Charge transfer between R6G and the N+ irradiated and vacuum annealed TiO 2 .

Probing of the SERS sensitivity of the irradiated TiO 2 samples shows a similar dramatic enhancement as observed for the irradiated WO 3 films. Specifically, the SERS spectra of the Ar+ irradiated and vacuum-annealed TiO 2 reveal that concentrations of R6G as low as (1×10-7 M) can be observed (Figure 8a); in contrast, for the pristine TiO 2 , the detection limit is only 1×10-2 M (Supporting Information Figure S10). Therefore, a 105 times of enhancement of SERS sensitivity is achieved.

Table 2. Performance of SERS active semiconductor materials and noble metal without ‘hot spot’ from literatures. Material

Colloidal Ag TiO 2 photonic microarray ZnO nanocrystals CuO nanocrystals Cu 2 O nanospheres W 18 O 49 nanowire MoO 3-x @MoO 3 nanosheets

Analyte Detection Excited Limit wavelength (M) (nm) -9 R6G 10 514 -6 MB 6×10 532

Reference

(45) (46)

4-Mpy

10-3

514.5

(47)

4-Mpy

10-3

514.5

(48)

4-MBA

10-3

488

(49)

R6G

10-7

532

(22)

MB

10-7

785

(23)

23

GaP particles InAs/GaAs quantum dots CuTe nanocubes H-Si nanowire Bulk TiO 2-x N y WO 3-x films ZnO 1-x

Cu Pc

---

514.5

(50)

pyridine

---

514.5

(51)

nile red

10-7

830

(52)

R6G R6G R6G R6G

10-6 5×10-8 1×10-7 1×10-7

532 532 532 532

(53) this work this work this work

To further demonstrate the universality of the ion beam method to enhance SERS sensitivity, we also used, another ion, N+, for irradiation. N+ irradiation can lead to the formation of energy levels of N-impurity and oxygen vacancies, which can further narrow the band gap of TiO 2 .26 Figure 8b shows the Raman spectra of Rhodamine 6G (R6G) with different concentrations of 1×10-6, 1×10-7, 5×10-8 and 1×10-8 M. Comparing these Raman spectra, it can be found the detection limit for the N+ irradiated and vacuum-annealed TiO 2 is 5×10-8 M, which is the lowest detection limit in the reported results for semiconductor SERS substrates (Table 2).22,23,45-53 The corresponding enhancement factor was evaluated to be 4.5×105 (see details in the Supporting Information). After N+ ion irradiation, nanorods with the diameter of about 23 nm and the length of about 100 nm were formed on the surface, which is due to the aggregation and growth of oxygen vacancies and N 2 bubbles.26 After annealing in vacuum, the nanorod structure did not change distinctly (Figure 8c and Figure S11). The nanorod structure with higher large specific surface area can improve the number of adsorbed molecules. However, we note that the Raman enhancement in the N+

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irradiated and vacuum-annealed TiO 2 is mainly attributed to the existence of oxygen vacancies instead of the increase of surface roughness and N impurity. After the vacuum-annealed TiO 2-x was oxidized in oxygen at 400 °C, its specific surface area did not change distinctly (Supporting Information Figure S11a-c) while the detection limit was only 1×10-2 M (Figure S11d). The SERS mechanism for the irradiated TiO 2 is shown in Figure 8d. The valence band and conduction band of rutile TiO 2 locate at -4.7 and -7.7 eV, respectively.54 Vo is referred to the oxygen vacancy energy level located at about 0.73 and 1.18 eV below the conduction band of the reduced rutile TiO 2-x .55 The localized states of the N impurity is about 0.75 eV above the valence band.56 Both additional possible resonances related to the oxygen vacancy and N impurity and the contribution of the nanorod structure contribute to the larger SERS enhancement. In order to further demonstrate the generality of the method, another semiconductor oxide (ZnO) was investigated as a target for SERS detection. Figure S12 shows Raman spectra of Rhodamine 6G (R6G) on the irradiated and annealed ZnO and the pristine ZnO. Under the same concentration 1×10-4 M, the irradiated and annealed ZnO performs the strong Raman peaks from R6G while no signal of R6G on the pristine ZnO was observed. Furthermore, the detection limit of the irradiated and annealed ZnO can reach 1×10-7 M. Thus, the irradiated and annealed ZnO also exhibited outstanding Raman enhancement.

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The irradiated substrates also have good stability. For a sample of WO 3 covered by R6G was placed in air for 45 days, the R6G characteristic peaks still could be clearly distinguished (Figure 9a). Good stability results from the formation of substoichiometric structures of WO 3-x (demonstrated by GIXRD). On the other hand, in previous reported papers, people used vacuum annealing to produce the reduced WO 3 or TiO 2 , where oxygen vacancies were only formed in the shallow surface. These vacancies are easy to be oxidized in air. However, for the vacancies produced by ion irradiation reported in this work, they distributed in the whole samples and are not easy to be oxidized by oxygen from air, which was demonstrated by XPS (Figure S13). Therefore, the prepared SERS substrates are stable. The reduced TiO 2-x also shows good stability (Figure 9b). The prepared substrate also shows good uniformity of SERS sensitivity (see Figure S14). Finally, the reported ion irradiation technique has great potential to provide a simple way to produce large-scale noble-metal-free SERS substrates with the diameter of 4 inches (Supporting Information Figure S15).

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Figure 9. (a) Raman spectra of R6G (1×10-6 M) obtained from the prepared WO 3-x substrate irradiated by 190 keV, 1×1017 Ar+ ions/cm2 and subsequently annealed in vacuum before and after being stored for up to 20 and 45 days in air. (b) Raman spectra of R6G (1×10-7 M) on the prepared TiO 2-x N y substrate irradiated by 1×1017 N+ ions/cm2 and vacuum-annealed before and after being stored for up to 3 months in air.

4. CONCLUSIONS In summary, we demonstrate that ion irradiation combined with vacuum annealing is a general and versatile method to produce large-area metal oxide semiconductor SERS substrates for highly sensitive molecular detection. We achieved detection limits as low as 10-8 M, which is an outstanding performance for semiconductors (the highest reported sensitivity so far) and even comparable to noble metals without ‘hot spots’. We demonstrated the irradiation-enhancement in three types of substrates, WO 3 , TiO 2 and ZnO, as well as using two types of ion beams, Ar+ and N+. Our systematic investigation of how the irradiation parameters and annealing conditions influence the SERS activity demonstrates that the SERS performance is attributed to the efficient charge transfer between semiconductors and molecules, resulting from two factors: (a) the formation of oxygen vacancy-associated electronic states in the band gap; (b) better crystallinity required to obtain the high electron mobility. Furthermore, N+ ion irradiation can also achieve N doping levels and nanostructures

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simultaneously as well as oxygen vacancies, which further improve the SERS enhancement. We believe that our strategy could be broadly applicable for the improvement of the SERS activity of a wide range of semiconducting oxides. Furthermore, this controlled technique is promising for industrial production of SERS substrates with large-scale, low cost, high stability and high sensitivity.

ASSOCIATED CONTENT Supporting Information Figures showing SEM images, optical images, absorption spectra, XPS spectra, Raman spectra and additional discussion, and tables showing resistivity and Hall measurements.

AUTHOR INFORMATION Corresponding Authors *Tel: +86-27-68752567. Fax: +86-27-68752569. E-mail: [email protected]. *Tel: +86-27-68752567. Fax: +86-27-68752569. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank the Prof. Hongxing Xu’s Group of Wuhan University for the support of Raman spectrum measurements and thank useful discussion with Prof. Yanfan Yan,

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Prof. Zhenyu Zhang and Hongxing Xu. This work was supported by the National Natural Science Foundation of China (11522543, 11475129, 51571153, 11375134), the Program for New Century Excellent Talents in University (NCET-13-0438), the Natural Science Foundation of Jiangsu Province, China (BK20161247), the Natural Science Foundation of Hubei Province, China (2016CFA080), and the Fundamental Research Funds for the Central Universities (2042017kf0194) for financial support.

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TOC A General Method for Large-Scale Fabrication of Semiconducting Oxides with High SERS Sensitivity Xudong Zheng, Feng Ren,* Shunping Zhang, Xiaolei Zhang, Hengyi Wu, Xingang Zhang, Zhuo Xing, Wenjing Qin, Yong Liu, Changzhong Jiang*

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