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Refractive Index Sensor Based on Leaky Resonant Scattering of Single Semiconductor Nanowire Yilun Wang, Baowei Gao, Kun Zhang, Kai Yuan, Yi Wan, Ziang Xie, Xiaolong Xu, Hui Zhang, Qingjun Song, Li Yao, Xin Fang, Yanping Li, Wanjing Xu, Jiasen Zhang, and Lun Dai ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00064 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017

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Refractive Index Sensor Based on Leaky Resonant Scattering of Single Semiconductor Nanowire

Yilun Wang,a,b Baowei Gao,a Kun Zhang,a,b Kai Yuan,a Yi Wan,a,b Ziang Xie,a Xiaolong Xu,a Hui Zhang,a Qingjun Song,a Li Yao,a Xin Fang,a Yanping Li,a Wanjing Xu,a Jiasen Zhang,a and Lun Dai *a,b

a

State Key Lab for Mesoscopic Physics and School of Physics, Peking University,

Beijing 100871, China. b

Collaborative Innovation Center of Quantum Matter, Peking University, Beijing

100871, China.

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ABSTRACT

Optical refractive index (RI) sensors, with the advantages of fast response, resistant to electromagnetic interference, and more options for signal collection, are widely used in biological field. Compared with the metallic nanoparticle RI sensors based on localized surface plasmon resonance (SPR), the nonmetallic nanomaterial RI sensors based on leaky resonant scattering, have the advantages of high sensitivity, large sensing volume, good reliability, and more suitable for biosensing. However, so far the study about nonmetallic nanomaterial RI sensor is less reported. In this work, we study the semiconductor nanowire (NW) RI sensors using CdSe nanowire as a case. The NW diameter and substrate effects on the sensitivity of the RI sensors are investigated experimentally and theoretically. The bulk wavelength sensitivity of a suspended NW can be as high as 235 nm/RIU (per refractive index unit), and the highest FOM (figure of merit) is 4.6. The large decay length (~100 nm) of NW sensor enables greater sensing volume than localized SPR sensor. Besides, we demonstrate that, by choosing proper diameter, we can tune the scattering efficiency peak into the optimum spectral region (600-900 nm) for biosensing. We also demonstrate that the semiconductor NW RI sensors have good reproducibility and reliability. Our work predicts that the semiconductor NW RI sensors have promising application in diverse sensing devices, especially in biosensing.

KEYWORDS: refractive index sensor, semiconductor nanowire, leaky resonant scattering, sensitivity, reliability


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Optical refractive index (RI) sensors ,with the advantages of fast response, resistant to electromagnetic interference,1–3 and more options for signal collection,4–6 are widely used and studied in biological field to measure concentration of the target analyte7–9 or detect binding events10 between the target analyte and its receptor. Among them, the propagating surface plasmon resonance (SPR) sensor based on prism and localized SPR sensor based on metal array can achieve extremely high sensitivity and figure of merit (FOM).7,11–13 But both of them lose the information about the spatial position of the sensed event. Single metallic nanoparticle sensor based on localized SPR can achieve spatially resolving nano-scaled refractive index changes, but suffering from much lower sensitivity and FOM. To date, many efforts have been made to increase the sensitivity and FOM of metallic nanoparticle sensors.14–18 Another option is to use nonmetallic nanomaterial to replace plasmonic materials in RI sensing or biosensing.19–21 Because of the optical field leaky nature for semiconductor nanomaterial with subwavelength feature size22,23, the scattering of it is sensitive to the ambient refractive index change. Single nanowire (NW) sensor can give high spatial resolution of RI change (~tens of nanometers) due to its geometry and field decay length. On the other hand, nanomaterial RI sensor can also have high sensitivity. Recently, Yang's group reported the semiconductor silicon nanosphere RI sensor, the intensity sensitivity and FOM of which is much higher than that of reported localized SPR sensor.19 Moreover, this kind of sensors can be small enough to be placed in a single cell to quantify chemical species and monitor the bioprocesses.24–29 Besides, the


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dark-field scattering microscopy, which uses much milder white light source instead of laser, would be more suitable for in-vivo sensing. Recently, the study about nonmetallic nanomaterial RI sensor has attracted increasing attention. In this work, we studied semiconductor NW RI sensors based on the leaky resonant scattering using CdSe NW as a case. We also experimentally and theoretically investigated the effect of both NW diameter and substrate on the sensitivity of the NW RI sensor. The result showed that the wavelength sensitivity of the sensor could be increased by using suspended nanowire, and the scattering efficiency (Qsca) peak position could be tuned into the window30 for biosensing by choosing proper NW diameter. In our case, the bulk RI wavelength sensitivity of a suspended nanowire was as high as 235 nm/RIU (per refractive index unit), and highest FOM is 4.6. We also demonstrated that the NW sensors had good reproducibility and reliability. Our work predicts that the NW RI sensors have promising application in diverse sensing devices, especially in nanobiosensors.

Results and Discussion


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Figure 1 Qsca spectra measurement and electric-field distribution of NW sensor. a) Schematic diagram of the experimental set-up for measuring Qsca spectra of NW under dark-field illumination. Inset: dark-field optical image of different diameters CdSe NWs on 285 nm SiO2/Si substrate, the green, yellow and orange NWs’ diameter is 62 nm, 128 nm, 185 nm, respectively. The scale bar in optical image represents 5 µm. b) Normalized Qsca spectra of different diameter CdSe NWs on 285 nm SiO2/Si substrate. Upper inset: dark-field optical images of the corresponding CdSe NWs. The scale bars represent 5 µm. Lower inset: SEM images of the corresponding CdSe NWs. The scale bars represent 300 nm. c) The electric field intensity profiles of the leaky modes resonance in NWs with different diameters. The black circles and lines indicate the NW/air and NW/substrate interfaces, respectively. d) and e) The near-field distributions along the white dashed line (121 nm) and black dashed line (193 nm) in (c), respectively. The electric field has an overlap with the ambient in the range of decay length.


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The CdSe NWs were synthesized via low pressure chemical vapor deposition (CVD) method31

(supporting

information

S1).

The

photoluminescence

(PL)

and

high-resolution transmission electron microscopy (HRTEM) results indicate that the as-synthesized CdSe NWs are with high quality and regular geometrical shape (Figure S1). The CdSe NWs were transferred onto 285 nm SiO2/Si substrates via a micro-manipulation method31,32 under the help of an optical microscope. Metallic markers were pre-fabricated on the SiO2/Si substrates for locating the NWs in later measurement. Figure 1a shows the schematic diagram of the measurement set-up for the NW RI sensor, as described in more detail in the Methods section. Under dark-field (DF) illumination condition, we can observe the CdSe NWs via the scattering light. The NWs with different diameters exhibit different colors as shown in the inset of Figure 1a. It is worth noting that herein the lengths of the NWs (~ dozens of microns) are much longer than the illumination light wavelength, and therefore have little effect on the DF optical color difference. Besides, quantum size effect is not expected in these NWs, since the Bohr exciton radius in bulk CdSe (~5.4 nm)33,34 is much smaller than the NW diameters. Usually, one can define the NW scattering efficiency Qsca as:35

− =

where and are the DF scattering light intensities from the NW and the background (BG) (the nearby substrate), respectively, and is the bright-field

(BF) reflecting light intensity from the background. The peak position of the


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scattering efficiency can be obtained by fitting the Qsca spectrum by Lorentz or a polynomial function (supporting information S2).36–38 The normalized Qsca spectra for the NWs are shown in Figure 1b. The Qsca peak position, which coordinates with the NW color in its DF optical image (the upper inset of Figure 1b), redshifts with NW diameter increasing. The diameters of the CdSe NWs were measured from their scanning electron microscope (SEM) images (the lower inset of Figure 1b). Quality (Q) factor of a NW optical cavity can be calculated with Q=f/∆f, where f and ∆f are the peak frequency and the full width at half maximum (FWHM) of the Qsca spectrum. Larger Q factor corresponds to longer resonance lifetime, which benefits in RI sensing.39 From Figure 1b, we can obtain the

Q factors to be about 5.4, 4.6, 4.3, and 3.9 for the NW with diameters of 45, 121, 193, and 252 nm, respectively. We can see that the non-suspended NW with smaller diameter has larger Q factor. This can be understood based on the theoretical analysis to be described below. In order to comprehensively understand the NW scattering efficiency, we adopt the finite-difference time-domain (FDTD), as detailed in the Methods section, to simulate the resonant modes in a NW (Figure S3). Usually, the resonant modes are denoted as TMmn or TEmn,40,41 where the integers m and n are the azimuthal and radial mode numbers, i.e., the effective number of wavelengths around the NW circumference and the number of electric field maxima along the radial direction, respectively. For sensing applications, the near-field confinement is the key feature. Figure 1c shows the total near-field intensity profile (E2) of the resonant mode for different diameters


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CdSe NWs. We can find that the scattering maxima corresponds to the same leaky mode (TM11) when the NW diameters are 121 and 193 nm. When the NW diameter is smaller (e.g. 45 nm), the nanowire cannot support TM11 mode, but can support TM01 mode. The difference between the TM and TE polarization is shown in Fig. S3a in the Supporting Information. When the diameter is larger (e.g. 252 nm), the TM11 leaky mode resonates with longer wavelength, but with relatively weaker electric field intensity according to FDTD electric field monitor. The near-field intensity of the nanowire-confined TM11 mode of 121 nm and 193nm diameter extends into the surrounding environment with decay length d ≈ 93 and 101 nm (Fig. 1d,e), which are larger than that of localized SPR sensors (∼10 nm).42,43 The analyte within the spacial range of decay length can be sensed by the NW contactlessly. Therefore, we can infer that the suspended NW, with larger fraction of electric fields overlapping with the analyte, benefits in RI sensoring.44 Related experimental result and theoretical analysis are discussed below.


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Figure 2 The Qsca spectra of NW sensors in different dielectric environment with and without substrate, respectively. a) Typical Qsca spectra of a CdSe NW (D=152 nm) on 285 nm SiO2/Si substrate in air (n =1.0), ethanol (n = 1.36) and glycol (n = 1.43). The illuminating light is un-polarized. The spectra are offset for clarity. The dashed lines indicate the same resonant modes. b) The Qsca peak position of both TM11 and TE01 resonant modes versus RI (n) relations. The sensitivities for TE01 and TM11 modes are about 93 and 43 nm/RIU, respectively, obtained from the linear regression analysis (the solid lines). FOM is also marked. c) Typical Qsca spectra of a suspended CdSe NW (D=150 nm) in different dielectric environment under un-polarized illumination. The spectra are offset for clarity. The dashed lines indicate the same resonant modes. d) The Qsca peak position of corresponding resonant modes versus RI (n) relations. The corresponding sensitivities, obtained from the linear regression analysis (the solid lines) are labelled in this figure.

For RI sensing, the CdSe NWs on substrate were placed in a chamber, which had an optical window (cover glass) for both light illumination and signal collection. Analyte


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liquid was pumped into the chamber. The optical image of the chamber and schematic diagram of the experimental set-up are shown in Figure S4. Typical Qsca spectra of a NW sensor (152 nm in diameter) measured in air (n =1.0), ethanol (n = 1.36), and glycol (n = 1.43) are shown in Figure 2a. The spectra are offset for clarity. The comparison of experimental and simulated Qsca spectrum is in supporting information S5. Due to the existence of the cover glass and the substrate, modes TE01 and TM11 become nondegenerate,19 as indicated by the dashed lines. It is apparent that the Qsca peak associated with TM11 mode redshifts, while that associated with TE01 mode blueshifts, as the ambient RI increases, agreeing with the previous experimental result based on nonmetallic quantum dots.19 Besides, we can see that the Qsca peak positions of both TM11 and TE01 resonant modes show a clear linear dependence on the RI of the analyte as shown in Figure 2b. The wavelength sensitivity45 Sλ of a NW sensor is defined as |∆λs/∆n|, where λs is the

Qsca peak wavelength. Using the linear regression analysis, we obtain the sensitivities to be about 93 and 43 nm/RIU for TE01 and TM11 mode, respectively. The FOM value46, defined as the sensitivity divided by the FWHM of Qsca peak is a criterion for assessing the biosensors. The FOM for TE01 and TM11 mode is 2.3 and 0.36. We further investigated the sensing behavior of suspended NWs. In order to do this, we transferred the NWs onto Si substrate with prefabricated micro-groove array (width =8 µm, height=1 µm, and spacing width=3.5 µm) on top. Figure 2c is the Qsca spectra of a suspended NW measured in different dielectric environments. Here, the NW is with similar diameter as that used in Figure 2a. The Qsca peaks in Figure 2c can


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be classified into three sets of resonant modes as indicated by the dashed lines. All of them redshift as ambient RI increases. The sensitivities of resonant modes TM21/TE11 (approximately degenerate) and TM11/TE01 (approximately degenerate) obtained from Qsca peak position versus RI relation (Figure 2d) are about 178 and 151 nm/RIU, respectively, higher than those of resonant modes TM11 and TE01 obtained from Figure 2b. The corresponding FOM is 4.6 and 3.6. It is worth noting that the Qsca peaks associated with mode TM01 locate in the biological window(600 ~ 900 nm), where the light has the maximum penetration depth in tissue.47 The corresponding wavelength sensitivity is about 126 nm/RIU(another sample result is shown in Figure S6) and the FOM is 1.8. This result predicts the potential application of the NW in biosensing.

Figure 3 The Qsca spectra of 205 nm suspended NW sensor in glucose solution with different concentrations (from 0 to 48%). Inset: the Qsca peak position versus RI (n) relations. The sensitivity (Sλ) is obtained from the linear regression analysis (the solid line).


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Herein, as an example, we demonstrate the performance of the suspended NW RI sensor for glucose detection, which is important for the diabetics. The results for glucose solution with different concentrations are shown in Figure 3a. The resonant mode TM11/TE01 is used here, because its wavelength locates in the biological window. The red shift of the resonant peak ranges from 5 to 19 nm as the glucose concentration is increased from 14% to 48%. From the linear dependence of Qsca peak position on the ambient RI shown in the inset of Figure 3, we can obtain the sensitivity to be about 235 nm/RIU, larger than that of nonmetallic arrary sensor.20 Herein, the FWHM (the definition of FWHM is described in Reference 1 supporting information) of the sensor is ~85 nm and the FOM is ~2.8.

Figure 4 The sensitivity and FOM dependence on diameter of NW sensors with and without substrate. Size effect on the sensitivity(a) and FOM (b) of both suspended CdSe NW (black and blue dots) and


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non-suspended CdSe NW (on 285 nm SiO2/Si substrate) (red and green dots). c-f) The electric field intensity profiles for TM11 mode of NWs with different diameters in air, with c, d) for non-suspended NW (on 285 nm SiO2/Si), and e, f) for suspended NW. The black circles and lines indicate the NW/air and NW/substrate interfaces, respectively.

The sensitivities and FOMs of both suspended and non-suspended NWs with different diameters are plotted in Figure 4a.b. The comparison of experimental and calculated sensitivities and FOM results is in supporting information S7. We can see that, compared with the non-suspended NW, the suspended NW has an overall higher performance. Besides, as the NW diameter increases, the sensitivities from modes TM21/TE11 and TM11/TE01 of suspended NW tend to increase, while that from mode TM11 of non-suspended NW tends to decrease. The latter is consistent with the corresponding Q factor versus NW diameter relation obtained from Figure 1b. In Figure 4b, the highest wavelength FOM we achieved is 4.6 which is larger than most single nanoparticle LSPR sensors (summary of related sensor works can be found in supporting information S8). In order to explain the higher sensitivities for suspended NW, we calculated the optical confinement factors in analyte, which can be derived from the electric field distributions of the resonant mode. The sensitivity of a NW RI sensor can be obtained by the first-order perturbation theory:48,49

δλ λ ≈ δ n na

∫ ε ∫ε Va

V

a

2

E dV 2

E dV

=

λ na

Ca

where λ is the wavelength of resonant mode, and E are respective the dielectric


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constant and electric field distribution in whole space V, , Va, na and Ca are the dielectric constant, volume, RI and confinement factor of the analyte, respectively. We can see that the sensitivity is proportional to both the resonant mode wavelength and the confinement factor. The electric field distribution for the TM11 mode of NWs are shown in Figure 4c-f. The calculated Ca values are about 23% and 21% for the suspended NWs with diameters of 150 and 200 nm, respectively, bigger than their counterparts (about 13% and 8%, respectively) for the non-suspended NW. The calculated sensitivities of the NW sensors have the relative relation of , where the superscripts refers to diameter, and the subscripts "su" and

"ns" refer to "suspended" and "non-suspended", respectively. This explains the observed experimental phenomena in Figure 4a.

Figure 5 The Qsca spectra of a NW RI sensor in air after being repeatedly used for ethanol sensing. The

number of circles is labeled in the figure. Inset: dark-field scattering images of the sensor in air after several circles of use.


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We also tested the reproducibility and reliability of the NW RI sensors by taking their dark-field images and re-measuring their Qsca spectra in air. Figure 5 plots the results for a NW RI sensor after having been repeatedly used for ethanol sensing. We can see that both the Qsca spectrum and position of the NW change little after the repeatedly immersing and redrying process. The limit of detection (LOD) is another important parameter for RI sensor, which measures the minimum resolvable RI change. LOD is defined as the ratio of the smallest scattering efficiency peak shift which can be measured in the presence of noise to the sensitivity of RI sensor. The resolution of the spectrometer in our experiment is 0.07 nm for 1200 l/mm grating, and the noise level for the system is about 0.2 nm as shown in Figure S9. Therefore, in our case, the smallest peak shift is determined by the noise level (i.e., 0.2 nm). Hence, the LOD can be estimated to be about 9 × 10−4 RIU for NW sensor with sensitivity of about 235 nm/RIU. According to the relation between concentration and refractive index,50 the minimum resolvable glucose concentration variation is estimated to be ~0.6%.

CONCLUSION

In summary, we have studied the semiconductor NW RI sensors based on the leaky resonant scattering. The results show that the CdSe NW sensors have the advantages of high sensitivity, nano-scaled spatial resolution, good reproducibility and reliability. We also demonstrate both experimentally and theoretically that the sensitivity of the NW RI sensor can be increased by using suspended NW, which has larger fraction of


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electric fields overlapping with the analyte. And suspended NW with higher FOM benefits in RI sensing. Our work predicts that the semiconductor NW RI sensors have promising application in diverse sensing devices, especially in biosensing.

METHODS

Optical Measurements. The measurement set-up includes an optical microscope (Zeiss Axio Imager, A2m) equipped a white light source (Zeiss Hal 100, 100 W), a charge-coupled device (CCD) camera and a spectrometer (Andor Shamrock 500i). An optical fiber (core diameter: 400 µm), with one end placed at the image plane of the microscope, collects the scattering light from the NW, and transmits it into the spectrometer. The spatial resolution of the Qsca spectrum, which depends on both the magnification of objective lens and the fiber diameter, is about 3.5 µm under 100× objective lens with a numerical aperture (N.A.) of 0.75. FDTD Simulation. In this simulation, an oblique-incident (50° determined by the NA of objective lens) total-field scattered-field (TFSF) source was used, and the mesh size was 1 nm. The NW, lying on a 285 nm SiO2/Si substrate, was taken as an infinitely long cylinder with a diameter of D and a complex RI of = + i,51 where the real part n is associated with the phase velocity, and the imaginary part k is the extinction coefficient. The complex RI of CdSe can be found in supporting information S10. The scattering efficiency under TM and TE polarized light illumination, denoted as "# and "$ respectively, can be defined as the ratio of the scattering cross-section of corresponding mode to the geometrical cross-section of


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the nanowire.52 The scattering efficiency %&' under un-polarized light illumination can be calculated as follow:53 1 %&' = ("$ + "# ) 2

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Synthesis and characterization of CdSe NWs. An example of the curve fitting of the Qsca Spectrum. Scattering modes of NW sensor. RI sensing measurement. Comparison of experimental and simulated simulated Qsca spectrum. Performance of the NW sensor in the window for biological tissue. Comparison of experimental and calculated sensitivity and FOM results. Summary of related sensor works. Noise level of the measurement system. CdSe complex refractive index. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] ORCID

Lun Dai: 0000-0002-6317-6340 Notes

The authors declare no competing financial interest.


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Acknowledgements This work was supported by the National Basic Research Program of China (Grant Nos. 2013CB921901 and 2012CB932703) and the National Natural Science Foundation of China (Grant Nos. 61521004, 61125402, 51172004, and 11474007).

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