Surface Structure and Current Transport Property of Boron and

Article pubs.acs.org/JPCC

Surface Structure and Current Transport Property of Boron and Phosphorus Co-Doped Silicon Nanocrystals Masato Sasaki, Shinya Kano,* Hiroshi Sugimoto, Kenji Imakita, and Minoru Fujii* Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *

ABSTRACT: Silicon (Si) nanocrystals (NCs) with high boron (B) and phosphorus (P) concentration shells are dispersible in polar solvents without organic ligands. In order to understand the mechanism of the solution dispersibility, the surface structure is studied by infrared absorption spectroscopy. It is shown that water molecules are adsorbed at the B−oxygen (O) bond sites on NC surface with high hydrogen bond strength, and thus B and P co-doped Si-NCs are a kind of hydrate containing large amounts of water molecules (Si-NC·xH2O). The current transport properties of Si-NC films made from the solutions are studied. It is found that the conductivity is very sensitive to the amount of adsorbed water molecules and changes by 8 orders of magnitude. The high affinity of the NC surface with water molecules is considered to be the origin of the high sensitivity.

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INTRODUCTION Colloidal dispersion of semiconductor nanocrystals (NCs) has been the subject of intensive research in recent years due to the potential applications in biomedical fields1−3 and solutionprocessed electronic devices.4−6 Among different kinds of colloidal NCs, silicon (Si) NCs are practically the most important due to the high biocompatibility2 and high compatibility with conventional semiconductor manufacturing processes. In any applications of colloidal Si-NCs, NCs should be dispersed in solution perfectly without agglomeration. To prevent the agglomeration, NC surface is usually capped by relatively long organic molecules, which prevent the agglomeration by the steric barriers.2,7−11 Another strategy to attain solution dispersibility to Si-NCs is inducing charges on the surface and preventing the agglomeration by electrostatic repulsions in polar solvents. In our previous work, we demonstrated that the formation of high boron (B) and phosphorus (P) concentration shells on the surface of Si-NCs induces negative surface potential and makes the NCs dispersible in polar solvents.12 A remarkable feature of the B and P co-doped Si-NCs is the high dispersibility in water in a wide pH range, which is essential for biomedical applications.13 Furthermore, the co-doped Si-NCs exhibit size-controllable photoluminescence (PL) in a wide wavelength range (0.85− 1.85 eV).13 One of the purposes of this work is to study the surface structure of B and P co-doped Si-NCs and elucidate the mechanism of the high water dispersibility. We show that boron−oxygen (B−O) bonds on the surface of NCs play a crucial role for the solution dispersibility; water molecules are adsorbed at the B−O sites with high hydrogen bond strength, and a kind of hydrate with a large amount of water molecules is formed. Another purpose of this work is the formation of thin films of NCs from the colloidal solution of B and P co-doped Si-NCs © 2015 American Chemical Society

and the investigation of the current transport properties. It has been demonstrated that controlling the surface structure is essential to determine the current transport property of Si-NC films.14 Pereira et al. showed 2 orders of magnitude increase of the conductivity, when the surface termination is changed from oxygen (O) to hydrogen (H).15 They also showed that tetrafluorotetracyanoquinodimethane (F4-TCNQ) adsorption results in n-type doping in Si-NC films.16 Holman et al.17 and Gresback et al.18 reported that H-terminated Si-NCs exhibit ntype transport properties without molecule adsorption. On the other hand, Cl-terminated Si-NCs show no gating behavior.18 Rastgar et al. found that adsorption of water molecules increases the conductivity an order of magnitude.14 In this work, we demonstrate that the conductivity of films made from B and P co-doped Si-NCs is extremely sensitive to water molecules and it changes at most 8 orders of magnitude. From the size dependence of the IR absorption spectra and the current transport properties, we show that high affinity of NC surface to water molecules due to B and P co-doping is the origin of the high sensitivity.

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EXPERIMENTAL PROCEDURE B and P co-doped Si-NCs were prepared by a co-sputtering method.12 First, Si-rich borophosphosilicate glass (BPSG) films were grown on thin stainless steel plates by sputtering Si, SiO2, B2O3, and P2O5 simultaneously in an rf-sputtering apparatus. The Si-rich BPSG films were then peeled from the plates, crushed into powder, and annealed in a N2 gas atmosphere at 1050−1250 °C for 30 min. The annealing process results in the growth of B and P co-doped Si-NCs in BPSG matrices. The average diameter of Si-NCs (Dave) is controlled by the Received: June 11, 2015 Revised: December 3, 2015 Published: December 8, 2015 195

DOI: 10.1021/acs.jpcc.5b05604 J. Phys. Chem. C 2016, 120, 195−200

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The Journal of Physical Chemistry C annealing temperature (Ta).19 The annealed powder was then etched in hydrofluoric acid (HF) solution (46 wt %), and codoped Si-NCs were extracted from the BPSG matrices. Finally, isolated Si-NCs were transferred to methanol. Figure 1a shows

the high-resolution TEM image (Figure 1f) corresponds to {111} planes of Si crystal. For electrical measurements of Si-NC films, Au electrodes (distance between electrodes, 200 μm; width, 5 mm) were made on fused silica substrates and Si-NC colloids were spincoated on the electrodes. The film thickness was 200−400 nm. The current voltage (I−V) characteristics were measured by a Keithley 236 source measure unit in dark. The measured temperature was changed from room temperature to 99 °C. The measurements were carried out in air, in vacuum, or in water vapor atmosphere.

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RESULTS AND DISCUSSION a. Characterization of Surface Structure. Figure 2a shows a typical IR absorption spectrum of a Si-NC film (Ta =

Figure 1. (a) Photograph of colloidal solution (methanol) of Si-NCs (Ta = 1200 °C). (b) SEM image of Si-NC (Ta = 1150 °C) film on Si wafer. The film thickness is about 650 nm. (c) Transmittance spectrum of Si-NC (Ta = 1150 °C) film. (d) Optical microscope image of Si-NC (Ta = 1150 °C) film. (e) TEM image of a monolayer Si-NC (Ta = 1200 °C) film. (f) High resolution TEM image of a Si-NC (Ta = 1200 °C).

a photograph of a methanol solution in which B and P codoped Si-NCs grown at 1200 °C are dispersed. Due to perfect dispersion of NCs, the solution is scattering-free and very clear. For infrared (IR) absorption measurements (PerkinElmer, Spectrum GX), Si-NC solution was drop-cast on a gold (Au)coated Si substrate. Absorption spectra were obtained by a reflection−absorption geometry with the incident angle of 5° in dry air. The measurement temperature was changed from room temperature to 90 °C. Figure 1b shows a scanning electron microscope (SEM) (Hitachi, S-3100H) image of a Si-NC film prepared by spincoating the solution on a Si wafer. The film is smooth and crack-free. It is optically flat, and the transmittance exceeds 90% in the near IR (NIR) range (Figure 1c). In the optical microscope image in Figure 1d, color gradience due to interference is clearly seen, supporting the high flatness of the film. Figure 1e shows a transmission electron microscope (TEM) (JEOL, JEM-200CX) image of a monolayer of NCs prepared by drop-casting the solution on a carbon-coated copper grid. No three-dimensional agglomerates are observed, and NCs are aligned two-dimensionally. The lattice fringe in

Figure 2. (a) IR absorption spectrum of Si-NC film (Ta = 1200 °C). (b) Integral intensities of the absorbance of Si−O−Si, Si−H, and B− O−B modes as well as that of the H−O−H bending mode normalized by the sum of the intensities as a function of the growth temperature. (c) Normalized IR absorption spectrum in the region of 1300−1550 cm−1 (B−O−B stretching mode) and (d) 1550−1800 cm−1 (H−O−H bending mode). (e) Model of the surface structure of Si-NC.

1200 °C). A major peak around 1065 cm−1 is Si−O−Si asymmetric stretching mode. Small peaks around 2100−2200 cm−1 and 1300−1500 cm−1 are assigned to the Si−H stretching and B−O−B stretching modes, respectively. In addition to these peaks, bending (1600−1750 cm−1) and stretching (2800−3800 cm−1) modes of water molecules adsorbed on the surface are observed. Peaks around 2900 cm−1 are C−H 196

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The Journal of Physical Chemistry C modes from residual methanol. No peaks assigned to Si−C bonds are observed. Unfortunately, the information on surface P atoms is not obtained from the IR absorption spectra because P−O−P stretching mode (1010−1070 cm−1) is overlapped with the strong Si−O−Si asymmetric stretching mode. The presence of P−O bonds is confirmed by XPS,20 although the number could be less than that of the B−O bonds because of smaller amount of P than that of B in Si-NCs.12,19 The number ratios of the Si−O−Si, Si−H, and B−O−B bonds, roughly estimated by taking into account the oscillator strengths of the vibration modes,21−24 are 80−85%, 13−16%, and 1−3%, respectively. Therefore, the surface of Si-NCs is mainly terminated by O and partly by H. It should be stressed here that average B concentration in Si-NCs estimated by ICP-AES (10−20 at %)19 is much higher than that estimated from IR absorption spectra. Therefore, a majority of B atoms doped in Si-NCs are in the nonoxidized states20 and only a fraction of them have bonding with O. This result is consistent with the XPS data.20 Figure 2b shows integral intensities of the absorbance of Si− O−Si, Si−H, and B−O−B modes as well as that of the H−O− H bending mode normalized by the sum of the intensities of these modes as a function of the growth temperature (see Supporting Information for the procedure of estimation). With increasing growth temperature, the intensity ratio of the Si− O−Si mode decreases while that of the B−O−B mode increases. The intensity of the H−O−H mode correlates positively with that of the B−O−B mode. This suggests that water molecules bind more likely to B−O−B than to Si−O−Si. This is consistent with previous reports that higher B concentration results in larger amount of adsorbed water in borosilicate films25 and that boron oxide tends to form hydrates with water molecules.26,27 Since O- or H-terminated Si-NCs are not dispersed in polar liquid, we speculate that B−O bonds contribute to the high solution dispersibility of co-doped SiNCs. The ratio of the Si−H mode is almost independent of the growth temperature. Figure 2c shows the IR absorption spectra of the B−O−B stretching mode of Si-NCs grown at different temperatures (Ta = 1050−1200 °C). The average size of NCs is shown in the figure. The spectra are broad covering the 1300−1500 cm−1 range and consist of several peaks. The vibration energy of B− O−B bonds is known to depend on the local environment, e.g., 1384 cm−1 in lithium borophosphosilicate, 1450 cm−1 in boron oxide, 1457 cm−1 in sodium borophosphate glass, etc.27−29 The observed complicated spectral shape suggests large distribution of the local microscopic environment of the B−O−B bonds. Probably B exists on the surface of Si-NCs in several different oxidation states, i.e., B3+, B2+, and B+. Figure 2d shows the IR absorption spectra of the H−O−H bending mode for Si-NCs grown at different temperatures. The broad spectra at room temperature indicate that the hydrogen bond strength is distributed in a wide range.30 When the growth temperature is Ta = 1050 °C (Dave = 3 nm), the maximum is around 1630 cm−1, which corresponds to that of liquid water.30 With increasing growth temperature, the higher wavenumber component grows and the maximum shifts to around 1700 cm−1. This shift indicates the increase of the hydrogen bond strengths.30 Figure 2e shows the model of the surface structure deduced from the IR absorption data. A major component of the surface is Si−O−Si, and there are small fractions of Si−H and B−O−B. In addition, B−O−Si may exist, although the mode at 930 cm−1

is not observed due to the overlap with the strong Si−O−Si mode. From theoretical calculation,31 P is considered to exist mainly in the back-bond of surface B atoms. We thus do not include P atoms in the surface model, although XPS data indicate the existence of a small amount of P−O on the surface. Water molecules are mainly adsorbed to the B−O sites. In Figure 3, IR absorption spectra of the H−O−H bending mode at room temperature and at 90 °C are compared. In all

Figure 3. IR absorption spectra (H−O−H bending mode) of Si-NC film measured at room temperature (black curves) and at 90 °C (red curves) grown at (a) 1050 °C, (b) 1150 °C, (c) 1200 °C, and (d) 1250 °C. (e) Ratio of integrated intensities of absorbance between H− O−H bending and Si−O−Si stretching modes as a function of growth temperature (Ta) measured at room temperature (■) and at 90 °C (▲).

the samples grown at different temperatures, the spectra shift to the higher wavenumber at 90 °C. No significant change of the intensity is observed in Figure 3e. These results suggest that at 90 °C water molecules do not escape from NCs and the hydrogen bond becomes stronger by heating than that at room temperature. Spectral shapes of Si−O−Si and B−O−B bonds are not modified by heating at 90 °C. As mentioned above, there is clear correlation between the intensities of the B−O−B stretching and the H−O−H bending modes. This suggests that water molecules are adsorbed preferentially at the B−O bond sites. Considering the high wavenumber of the H−O−H bending modes and the fact that water molecules are not removed at 90 °C in Figure 3e, they are not simply adsorbed. A correct description of B and P codoped Si-NCs may be Si-NC hydrates (Si-NC·xH2O), similar 197

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The Journal of Physical Chemistry C to the case of hydrated boron oxide (B2O3·xH2O)27 and hydrated borates.26 To our knowledge, hydrates of luminescing semiconductor NCs with single nanometer size have not been reported, although there are many kinds of metal oxide particle hydrates.32−35 b. Current−Voltage Characteristics. Figure 4a shows I− V characteristics of films of B and P co-doped Si-NCs grown at

(Figure 4c) is about 1 order of magnitude higher than that in air (Figure 4b). In Figure 4d, current at a fixed voltage (50 V) is plotted as a function of the water vapor pressure. The background current of the measurement setup is 10−13 A. Below 100 Pa, it is smaller than the background level and it continues to increase to the saturated vapor pressure (3.36 kPa). From 100 Pa to 3.36 kPa, the current changes by 8 orders of magnitude. Although similar current modulation by molecule adsorption/desorption has been reported in other types of Si-based nanostructures,14,15,36−38 to our knowledge, the observed current modulation is the largest ever reported. In Figure 5a, I−V characteristics at different temperatures are shown. The conductivity is sensitive to the measurement

Figure 5. (a) I−V characteristics of a Si-NC film in air. The measurement temperature is changed from room temperature to 93 °C (Ta = 1050 °C). (b) Current value at fixed electric field (2.5 kV/ cm) as a function of measurement temperature.

Figure 4. (a) I−V characteristics of a Si-NC film at room temperature in air. (b) Current value at fixed electric field (2.5 kV/cm) when evacuation of and filling of air are repeated. Lower panel: degree of vacuum (Ta = 1050 °C). (c) Current value at fixed electric field (2.5 kV/cm) when evacuation of and filling of water vapor are repeated. Lower panel: degree of vacuum (Ta = 1050 °C). (d) Current value at fixed electric field (2.5 kV/cm) as a function of water vapor pressure.

temperature and starts to decrease even when the temperature is raised only 6 °C. In Figure 5b, the value of current at 2.5 kV/ cm is plotted as a function of the measurement temperature. It decreases at maximum by 4 orders of magnitude from room temperature to 99 °C. The current modulation by heating and cooling is not fully reversible; the current value recovers only slightly (10−9−10−10 A) after cooling the samples. Note that the surface is not oxidized by heating as confirmed by IR absorption spectra (see Supporting Information). Current modulation by molecule adsorption/desorption has been observed in different kinds of Si-based nanostructures and is usually explained by electron (hole) accumulation by band bending. In H-terminated silicon on insulator (SOI), the conductivity increases by water adsorption and it is explained by electron accumulation by band bending.36,39 In Si nanowires (Si-NWs), hole accumulation due to adsorption of water molecules at surface dangling bonds is reported.38 In Clterminated Si-NCs, free carriers are generated in Si-NCs by adsorbing 2-butanone.40 In this case, generation of free carriers is shown by free carrier absorption in IR absorption spectra. In the present samples, we did not observe the free carrier absorption, and thus carrier generation by water adsorption is not the origin of the current modulation. An essential difference between the present NC films and SOI and Si-NWs is that current transport is controlled by electron tunneling between NCs. In this work, the distance between electrodes is 200 μm, and thus tens of thousands of SiNCs are connected in between electrodes. Therefore, a small difference of tunneling resistance between Si-NCs results in significant change of the conductivity. A possible explanation of the observed drastic increase of the current by water adsorption

different temperatures at room temperature in air. In many cases, the I−V curve shows hysteresis and the current value changes about ±35% during one cycle of the I−V measurement (not shown). When the growth temperature is below 1200 °C, I−V characteristics are almost independent of the growth temperature. The conductivities are in the range of 10−6−10−7 S/cm, which are comparable to those reported for Hterminated Si-NC films.15 For the sample grown at 1250 °C, the conductivity is several orders of magnitude smaller than that of the other samples. The I−V characteristics are sensitive to the measurement environment. In Figure 4b, the value of the current at a fixed electric field (2.5 kV/cm) for the sample grown at 1050 °C is shown. The measurement atmosphere is changed from air to vacuum periodically. The degree of vacuum is shown in the lower panel. The current value changes steeply following the vacuum pressure in more than 6 orders of magnitude. Similar reversible change of the conductivity is observed for all Si-NC samples grown at different temperatures. For the sample grown at 1250 °C, the amplitude of modulation is smaller than that of the other samples because of the lower current in air. A possible origin of the observed current modulation is adsorption and desorption of water molecules in air.14,15 Figure 4c shows the result of a similar experiment for water vapor. After a few evacuation/filling cycles, the current values in water vapor 198

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is that it decreases the height of the tunneling barrier. Another possible model is ionic conduction (proton conductivity), which is proportional to the adsorbed water molecules. For example, in silica gel, clear correlation between the conductivity and the amount of adsorbed water molecules is reported.41

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CONCLUSIONS The analysis of IR absorption spectra revealed that in B and P co-doped Si-NCs, a small fraction of surface B atoms are oxidized and water molecules are adsorbed at the B−O bond sites with high hydrogen bond strength. The B and P co-doped Si-NCs are thus considered to be a kind of hydrate containing a large amount of water molecules (Si-NC·xH2O). The high affinity of B and P co-doped Si-NCs with water molecules appears as very high sensitivity of current transport properties of NC films to adsorbed water molecules. The largest conductivity change observed is 8 orders of magnitude. The very high sensitivity of the conductivity suggests that the Si-NC films can detect tiny polar molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05604. Determination of integral intensities of IR spectra and IR spectra of Si−O−Si modes at room temperature and at 90 °C (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S.K.: e-mail, [email protected]; phone, +81-78-8036291. *M.F.: e-mail, [email protected]; phone, +81-78-8036081. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by KAKENHI (Grant 26886008), 2014 JSPS Bilateral Joint Research Projects (Japan−Czech Republic), Visegrad Group (V4)-Japan Joint Research Project on Advanced Materials “NaMSeN”, and Casio Science Promotion Foundation.

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DOI: 10.1021/acs.jpcc.5b05604 J. Phys. Chem. C 2016, 120, 195−200

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DOI: 10.1021/acs.jpcc.5b05604 J. Phys. Chem. C 2016, 120, 195−200