Introduction

Since the discovery, one-dimensional (1D) carbon nanotubes (CNTs) have drawn considerable attention on the design and fabrication of molecular optoelectronic nanodevices because of their unique and tunable electronic, optical and chemical properties1,2,3. Significant progresses have been made towards the preparation of a series of photo-responsive CNT-based nanocomposites such as photochromic molecules/polymers, organic fluorescent dyes and nanocrystals4,5,6,7,8. Among these, quantum dots (QDs)-anchored CNT nanohybrids combining high light-absorption and efficient charge transport based on their unique nanostructures attract an increasing amount of attention9,10. This nanocomposite not only enables multiple exciton generation (MEG) of QDs but also provides large QD/CNT interfaces and 1D conductive pathways favouring photo-induced charge transfer and transport11. Therefore, QD/CNT nanocomposites are regarded as one of ideal candidate materials for the fabrication of optoelectronic devices, such as photodetector, photoswitch and solar cells9,12,13.

In the past few years, several strategies were presented to couple various QDs to nanotubes with different linkages9,14,15,16,17. One of effective coupling approaches is through the electrostatic interaction between negatively and positively charged properties of two components. Previous studies showed that the surface charges of CNTs generated by tailoring chemical structures could be tuned using different polyelectrolytes (PE)9. Water-dispersed PE-grafted CNTs become a hydrophilic scaffold for the surface immobilization of opposite charged QDs by the electrostatic self-assembly. The electrostatic interaction between QDs and CNTs is controlled by the diameter, dispersion, charge density and packing density of QDs on the surface of PE-grafted CNTs. Giersig et al.18 reported the fabrication of QD/CNT heterostructures by the electrostatic interaction. Multi-walled carbon nanotube (MWNT) coated by poly(allylamine hydrochloride) (PAH) was decorated by ZnO, CdSe and CdSe-CdS nanocrystals. Nanocomposites with a highly defined morphology provided good separation and stability in aqueous solution. Zhu et al.19 found that PAH-functioanlized MWNT electrostatically attached by CdSe QDs exhibited high electrochemiluminescence (ECL) intensity, good biocompatibility and high stability. Jia et al.20 also presented a layer-by-layer assembly of the cationic PE-grafted MWNT with oligodeoxynucleotide-tagged CdTe QDs by the electrostatic interaction. This layered nanostructure delivering therapeutic genes into the nuclei of cells showed efficient intracellular transport, strong cell nucleus localization and high delivery efficiency.

Recently, a variety of UV-vis active aqueous QDs (e.g., ZnO, CdSe, CdS, CdTe.) has been utilized to decorate PE-grafted CNTs by the electrostatic interaction9,18,21. However, near-infrared (NIR) fluorescent lead sulfide (PbS) QDs with a narrow band gap (0.41 eV) and a large exciton Bohr radius (18 nm) have hardly been incorporated into CNT-based nanocomposites by the electrostatic assembly because of the structural inhomogeneity. PbS QDs prepared in aqueous solution exhibited a long-wavelength emission, a wide size distribution and poor storage stability22, leading to the decrease in optical properties. While monodispersed PbS QDs with good optical absorbance synthesized in colloidal organometallic solution were difficult to decorate aqueous PE-grafted CNTs because of incompatibility between two phases. The compatibility between QDs and CNTs can be improved by the phase transfer of PbS from an organic to an aqueous solution. Chemical structure controlled by the ligand exchange on the surface not only enables PbS to mono-disperse in water but also enhances the photoluminescence quantum efficiency. Wang et al.23 showed the ligand exchange of trioctylphosphine oxide-coated QDs with polydimethylaminoethyl methacrylate homopolymers. The modified QDs are soluble in polar media and retain 70% of original photoluminescence (PL) quantum yield. Mono-dispersed PbS in an aqueous solution by the phase transfer combining a narrow size distribution and a high quantum yield can be utilized for a high-quality assembly of PbS/CNTs by the electrostatic interaction.

In this paper, a versatile noncovalent method was presented to prepare the electrostatic assembly of PbS/MWNT nanostructures after the phase transfer of PbS QDs from an organic to an aqueous solution (Figure 1). The microstructures and chemical structures of PbS QDs and PbS/MWNT hybrids were characterized by high-resolution transmission electron microscopy (HRTEM) and Fourier-transform infrared (FT-IR) spectroscopy, respectively. UV-vis-NIR absorption and PL measurement were also used to study photophysical properties. Moreover, a two-terminal device using PbS/MWNT nanostructures was fabricated. Results indicated that PbS/MWNT nanocomposites showed a switching photocurrent and good stability under the illumination of light at different intensities.

Figure 1
figure 1

Illustration of the preparation of PbSCys/MWNT nanocomposites by electrostatic interaction.

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Results

Morphologies of oleic acid (OA)-capped PbS (PbSOA) and L-Cysteine (Cys)-capped PbS (PbSCys) QDs were observed by TEM. Figure 2a shows that PbSOA QDs prepared by a colloidal organometallic chemistry method are mono-dispersed in toluene with a diameter of 6.6 ± 0.3 nm (Figure S1) and the atomic ratio of Pb to S atoms is 29/14 according to energy dispersive X-ray spectrometry (EDS) (Figure S2). The electron diffraction (ED) pattern (the inset of Figure 2a) indicates that well-crystallized PbS shows the cubic rock salt structure, which is also confirmed by X-ray diffraction (XRD) pattern (Figure S3). HRTEM (Figure 2b) image reveals that PbSOA QDs exhibit a high-crystalline structure with an interplanar distance of 0.293 nm (the inset of Figure 2b), corresponding to the (200) plane of the fcc lattice. PbSOA QDs in colloidal organometallic solution were prepared using a long-time and high-temperature reaction, which facilitates the homogeneous nucleation and the growth of QDs. As a result, PbSOA QDs are capped by OA ligands with a long alkyl chain, preventing QDs from being oxidzed. Thus, as-prepared PbSOA QDs show a good stability and mono-dispersed size distribution.

Figure 2
figure 2

TEM images of (a) mono-dispersed PbSOA and (c) PbSCys QDs with the inset of corresponding fast fourier transforms (FTTs).

HRTEM images of (b) PbSOA and (d) PbSCys QDs with the inset of the lattice fringes of an individual QD.

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TEM images of water-dispersed PbSCys QDs after the phase transfer through the ligand exchange were given in Figure 2c and 2d. PbSCys QDs also show a uniform mono-dispersion and a high-crystallinity structure (the inset of Figure 2d) after the phase transfer from toluene to water (Figure 2c). During the phase transfer, the ligands on the surface of PbSOA QDs changed from OA to Cys, which thus stabilizes QDs in water with a narrow size distribution. The ligard exchange from toluene to water does not affect the dispersion of QDs and only leads to the slight variation of diameters. Compared with PbSOA (6.6 ± 0.3 nm), PbSCys exhibits a decrease in diameter (5.3 ± 0.4 nm) (Figure 2d and S4). According to previous studies24,25, the decreased size was attributed to the etching of QDs surface by the ligand exchange, during which oleic groups were replaced by Cys. Simultaneously, some Pb atoms and/or S atoms might be carried away from the core because of the replacement of OA by Cys during the phase transfer. The ligand exchange on the surface of PbSCys QDs is also confirmed by EDS (Figure S5) and the increase in atoms.

TEM images of the electrostatic assembly of PbSCys/MWNT nanocomposites were shown in Figure 3 and S6. MWNT with the diameter of 15–25 nm shows an individual dispersion due to the reduced van der Waals attraction by the surface modification. Rough surface of the sidewalls (the inset of Figure 3a) is attributed to the damage of carbon conjugated structures of nanotubes. The fracture of carbon-carbon double bonds by the oxidation of acids results in the generation of many oxygen groups such as carboxyl and carbonyl. Despite being hardly distinguished in TEM images, these side groups are useful for the electrostatic interaction between poly(diallyldimethylammonium chloride) (PDDA) and MWNT. The electrostatic assembly between negatively charged PbSCys QDs and positively charged PDDA grafted MWNT (PD-MWNT) was shown in Figure 1. Mono-dispersed PbSCys QDs with a uniform size distribution are uncovalently anchored on the sidewalls of MWNT without free QDs on the grids by the electrostatic interaction. The packing density of PbSCys QDs was tuned by the volume ratios of PbSCys to MWNT (Table 1). PbSCys/MWNT5:1 and PbSCys/MWNT1:1 nanocomposites were prepared by weight ratios of PbSCys to MWNT of 5:1 and 1:1, respectively. Compared with PbSCys/MWNT1:1 (Figure 3b), PbSCys/MWNT5:1 nanocomposite (Figure 3c) shows a remarkable increase in packing density of homogeneously dispersed QDs on MWNT because of a high proportion of PbS QDs in the assembly. The crystalline structures of PbSCys QDs were further observed by HRTEM (Figure 3d). PbSCys QDs on the sidewalls show the diameter of 5.5 nm, which is consistent with that of PbSCys QDs dispersed in water after the phase transfer. Recent works26,27 demonstrated that the efficiency of charge transfer could be directly affected by decreasing the length of ligands, which are used to avoid the self-agglomeration. Therefore, this uniform layer-by-layer electrostatic assembly of PbSCys/MWNT with high affinity interaction facilitates charge transfer at the interface between QDs and nanotubes.

Table 1 The detailed volumes of PbSCys and PD-MWNT solution for the electrostatic assembly
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Figure 3
figure 3

TEM image of (a) Water-dispersed MWNT with the set of high resolution (scale bar is 10 nm).

(b) PbSCys/MWNT1:1 and (c) PbSCys/MWNT5:1. (d) HRTEM image of PbSCys QDs closely attached to MWNT.

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Chemical structures of PbS QDs and PbSCys/MWNT nanocomposites were studied by FT-IR spectra. PbSOA QDs (Figure 4a) show four characteristic peaks of OA including C = O stretching vibration at 1735 cm−1, C-O stretching vibration at 1118 cm−1 and −CH2 asymmetric stretching vibration at 2924 cm−1 and symmetric stretching vibration at 2854 cm−1. Compared with PbSOA, the ligand exchange from OA to Cys during the phase transfer is confirmed by a new peak at 1610 cm−1 and a broad peak at 1072 cm−1 corresponding to NH2 deformation vibration and C-N stretching mode, respectively in PbSCys (Figure 4b). Meanwhile, no characteristic peak of S-H between 2550 and 2680 cm−1 indicates Cys molecules are chemisorbed as thiolates28 by PbS QDs (Figure 4b).

Figure 4
figure 4

FT-IR spectra of (a) PbSOA, (b)PbSCys QDs, (c) MWNT, (d) PD-MWNT and (e) PbSCys/MWNT nanocomposites.

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The grafting of oxygen groups such as -COOH and -OH on acid-treated MWNT is indicated by several peaks including νOH at 3442 cm−1, νC = O at 1654 cm−1, νC-OH at 1390 cm−1, δOH at 1060 cm−1 in Figure 4c. These groups on the sidewalls favor the electrostatic interaction between MWNT and PDDA with opposite charges. Three peaks of −CH2 bending vibration at 1400 and 947 cm−1 and C-N stretching vibration at 1085 cm−1 suggest that MWNT is wrapped by PDDA by the electrostatic interaction. The attachment of PbSCys QDs on MWNT by the electrostatic self-assembly is demonstrated by multiple peaks in the curve of PbSCys/MWNT (Figure 4e) including NH2 deformation vibration (1610 cm−1), C-OH stretching vibration (1400 cm−1), −CH2 bending vibration (1400 cm−1 and 947 cm−1) and C-N stretching vibration (1072 cm−1). Furthermore, asymmetric and symmetric −CH2 stretching bands in PbSCys/MWNT show large red-shifts to 2927 and 2852 cm−1, respectively compared with PD-MWNT at 2976 and 2922 cm−1. As illustrated in previous studies, −CH2 stretching vibration was an identifying indicative of the order of alkyl chains29. Thus, red-shifted bands of −CH2 stretching indicates the increased order of alkyl chains resulting from the electrostatic adsorption of PbSCys QDs on MWNT. Results reveal that PbSCys QDs mediated by the phase transfer are anchored on the sidewalls of MWNT by the electrostatic assembly.

NIR absorption spectra of (a) PbSOA in toluene, (b) PbSCys QDs, (c) PbSCys/MWNT5:1, (d) PbSCys/MWNT1:1 and (e) MWNT in water were shown in Figure 5. PbSOA QDs with the diameter of 6.6 nm show an absorption band with the maximum peak (λmax) at 1476 nm in NIR region. After the phase transfer, a dramatic blue shift of λmax (1264 nm) is obtained for PbSCys QDs because of a combining effect of the decreased size of PbSCys (5.3 nm) and the change of ligands capped on the surface24,25. Compared with water-dispersed MWNT (Figure 5e), PbSCys/MWNT (Figure 5c and 5d) exhibits an increase in NIR absorbance in the range from 1700 to 900 nm because of the attachment of NIR-active mono-dispersed PbSCys QDs. However, the absorption edge and the peak of band of PbSCys QDs are not distinguished clearly in the NIR region for two curves of PbSCys/MWNT nanocomposites. This band-edgeless absorption was also observed in a series of QDs attached on the surface of CNTs including CdSe18, PbS27 and CdS31. According to previous studies, this result was attributed to the charge diffusion or electronic interaction between QDs and MWNT in the ground state at the interface. The featureless band-edge absorption in NIR region was an indicative of intimate contact between PbSCys QDs and MWNT, which enables efficient photo-induced interfacial charge transfer.

Figure 5
figure 5

NIR absorption spectra and of (a) PbSOA in toluene, (b) PbSCys QDs, (c) PbSCys/MWNT5:1, (d) PbSCys/MWNT1:1 and (e) MWNT in water with the inset of enlarged spectra at the corresponding wavelength.

Top inset: optical images of PbSOA QDs (left) in toluene and (right) in water after the phase transfer.

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Furthermore, NIR absorption of PbSCys/MWNT can be further tuned by packing density of PbSCys QDs on the surface of MWNT. It can be seen that NIR absorption of PbSCys/MWNT5:1 nanocomposite is higher than that of PbSCys/MWNT1:1 due to high density of PbSCys QDs on the sidewalls. Moreover, PbSCys (Figure 5b) and PbSCys/MWNT (Figure 5c and 5d) in water exhibit small bumpers around 1500–1800 nm owing to strong water resonance, which is hardly eliminated by measuring the baseline30. This analysis is confirmed by the result that no bumper is found in the absorption of PbSOA QDs in toluene (Figure 5a).

Figure 6 displays PL spectra of (a) PbSOA in toluene, (b) PbSCys QDs, (c) PbSCys/MWNT5:1 and (d) PbSCys/MWNT1:1 in water. PbSOA QDs shows a strong emission peak (λem) at 1554 nm (Figure 6a). Compared with PbSOA, a remarkably reduced intensity of λem at 1309 nm with a blue-shift by 245 nm is observed in PbSCys QDs (Figure 6b). According to previous studies24,32, PL intensity showed a strong surface dependence and thus was sensitive to the structure of ligands. Furthermore, during the phase transfer, oxidative etching of QD surface occurred because of the existence of oxygen in solution25. Consistent with NIR absorption, the blue-shifted emission band arises from combining effects of the decreased size from 6.6 to 5.3 nm, the re-distribution of electronic density and the increased confinement energy, resulting from the formation of strong Pb-thiol bond on QDs by the ligand exchange33. Moreover, when anchored on the surface of MWNT, PbSCys QDs show a remarkable quenching of blue-shifted luminescence. It can be seen that PL intensity at λem of PbSCys/MWNT5:1 and PbSCys/MWNT1:1 decrease by 20.3% and 28.6% respectively compared with PbSCys QDs. Strong quenching of PL is attributed to the presence of alternative nonradiative decay pathways favoring charge transfer from the conductive band of PbS QDs (donor) to the empty electronic state of MWNT (acceptor) at the interface18,21,34. Charge transfer from PbSCys QDs to MWNT is facilitated by their respective band energy levels (Figure 7d) based on the intimate contact through the electrostatic interaction. The interaction is also confirmed by a blue-shifted emission band of PbSCys/MWNT nanocomposites. The λem of PbSCys/MWNT5:1 and PbSCys/MWNT1:1 blue-shift to 1027 and 1009 nm responsively from 1309 nm of PbSCys QDs. This blue-shifted emission bands of QD/MWNT nanocomposites are consistent with the reports by Wang et al.25,35 recently. PbS QDs capped with the oleylamine ligands attached on the surface of MWNT exhibit a remarkable blue-shift of emission band due to the electronic interaction between PbS and MWNT at the interface. In our hybrid, PbSCys QDs are absorbed on the surface of MWNT by the electrostatic interaction between the ligands and PDDA. As a result, the blue-shifted band is attributed to the electronic interaction between PbSCys and MWNT by changing electronic density and the confinement energy of PbSCys rather than decreased QDs sizes. TEM images show that PbSCys QDs decorated on MWNT shows the diameter of 5.5 nm, which is the same to that of dispersed PbSCys in water after the phase transfer. This analysis is also confirmed by the result that PbSCys/MWNT5:1 with no changes in QDs size exhibits a larger blue-shift of λem than that of PbSCys/MWNT1:1.

Figure 6
figure 6

PL spectra of (a) PbSOA in toluene, (b) PbSCys QDs, (c) PbSCys/MWNT5:1 and (d) PbSCys/MWNT1:1 in water under the excitation of visible light at 465 nm.

Inset shows the enlarged spectra at the corresponding wavelength.

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Figure 7
figure 7

Photoresponse of a two-terminal device based on PbSCys/MWNT5:1 nanocomposites.

(a) Representative I-V curves of PbSCys/MWNT5:1 in dark and under light irradiation of 2.03 mW/cm2. Top left inset: scheme illustration of a two-terminal device and right bottom inset: SEM image of PbSCys/MWNT5:1 deposited on a Si/SiO2 substrate. (b) Photocurrent response of a photoswitch during seven ON/OFF cycles under light irradiation of 0.87, 1.76 and 3.85 mW/cm2. (c) Photo sensitivity of a photoswitch during 20 cycles under light irradiation of 0.87, 1.76 and 3.85 mW/cm2. (d) Energy level diagram adjusted in relation to the vaccum level of PbSCys/MWNT hybrid. Arrow indicates electron transfer from PbS to MWNT under light illumination.

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Discussion

Photoresponse of a two-terminal photoswitch based on PbSCys/MWNT5:1 hybrid was investigated by drop-casting hybrid solution onto source/drain channels on a Si/SiO2 (450 nm) substrate. Schematic depiction of a two-terminal device was shown in upper inset of Figure 7a. SEM image (the right bottom inset of Figure 7a) displays 3D interconnected networks of PbSCys/MWNT5:1 in the channel of devices, favoring charge mobility from source/drain to gate. Figure 7a shows typical current-voltage (I-V) curves of PbSCys/MWNT5:1 nanocomposites in dark and under light irradiation of 2.03 mW/cm2. Compared with I-V response in dark, a remarkable decrease in current is obtained under light illumination. The decreased current returns to its initial value in dark when the light is switched off. Previous studies demonstrated that the device based on pure MWNT with the same structure shows no increase in photocurrent when the light was switched on/off11,31. Thus, the decrease in photocurrent of PbSCys/MWNT5:1 nanocomposite arises from the photo-induced electron transfer between PbS and MWNT. Photoactive on/off switching characteristics of the device under light irradiation of 0.87, 1.76 and 3.85 mW/cm2 at 2 V source-drain bias (Vds) was further studied in Figure 7b. The light was turned on and off for 40 s. Each cycle of the current consists of three transient regimes – a sharp decrease and constant state under the illumination and a fast relaxation to dark value when the light is off. The response (light on) and recovery time (light off) of photocurrent in every cycle are shown in Figure S7. PbSCys/MWNT5:1 nanocomposite exhibits a slight increase in response time and recovery time as the increasing number of cycles. The response time of PbSCys/MWNT5:1 irradiated by the light of 3.85 mW/cm2 changes from 9.0 s (the first cycle) to 12.9 s (the tenth cycle). The increasing response time of photocurrent upon cycling was also observed by previous studies15,36,37. It also can be seen that the response time and recovery time of PbSCys/MWNT5:1 irradiated by the light of 1.76 and 3.85 mW/cm2 is longer than that of 0.87 mW/cm2. This result indicates that a great amount of electrons is not excited and transfers to MWNT at the interface by a low-intensity light. Thus, the photoresponse of PbSCys/MWNT can be adjusted by the output energy of irradiation light.

Figure 6b shows that the on/off switching current was tuned by the intensity of light illumination. Photocurrent (Iph) and photo sensitivity (P) are evaluated by the equation (1) and (2), respectively.

where Id is the current in dark and Il is the current under illumination. As shown in Figure 7c, PbSCys/MWNT5:1 exhibits an increase in photo sensitivity under illumination of light at a high power density. Specifically, the photoswitch shows a high Iph of 18.3 μA and P up to 31.3% under the light of 3.85 mW/cm2 compared with 3.5 μA and 6.0% at 0.87 mW/cm2. Moreover, the device holds a good reversibility and cycling stability of photoresponse. Photo sensitivity shows no decay during 20 cycles under illumination of light at three intensities. Photoresponse and cycling performance of PbSCys/MWNT5:1 outperform many QD/carbon-based nanocomposites reported13,14,32. The stable and reversible photoresponse tuned by light intensity is critical for high-performance sensors and rewritable memory devices.

Generally, the increase in Iph of the switch using QD/CNT nanocomposites was mainly attributed to charge transfer between QDs and CNTs at the interface. In this work, the delocalized electron-hole pairs over PbS QDs created by incident light excitation dissociate at the interface between PbS and MWNT38. This interaction results in electron transfer from PbS to MWNT11,15, which is facilitated by the intimate contact between QDs and nanotubes by the electrostatic assembly. Thus, electron-hole recombination on the nanotube (hole transporter) reduces Il due to the decreased hole concentration in MWNT network, leading to the increase in Iph. Besides, according to previous studies11,16, holes in PbS QDs could also be annihilated by the reaction in the environment. Electrostatic interaction (gate effect) between PbS and MWNT drives nanotubes into off-state, which also contributes to the decrease in Il. The analysis is confirmed by the result that Iph is tuned by illuminated light of different densities. Excited by a high-density light, an increasing amount of electron-hole pairs is generated for the dissociation and transfer at the interface between PbS and MWNT. Thus, more electrons created by PbSCys are recombined and annihilated by holes on the nanotubes, resulting in a lower hole concentration. Meanwhile, the gate effect is also significant under illumination of light at high density. Consequently, combining effects of hole recombination and gate effect result in a remarkable increase in Iph.

Electron transfer in PbSCys/MWNT nanocomposites is illustrated by band energy diagram (Figure 7d). The lowest unoccupied molecular orbital (LUMO) energy of PbSCys QDs is -4.32 eV calculated according to reference37,39 and band gap is 0.86 eV from the formula reported by Hens et al.40 and thus the highest occupied molecular orbital (HOMO) level is -5.18 eV. The favorable energy band alignment is advantageous to exciton dissociation and transfer at the interface. Under illumination, electron in the ground state of PbS transits to the excited state by absorbing photons. Electron-hole pairs formed on the surface of QDs dissociated at the interface. Electron transfer from PbS to MWNT (Fermi level is 4.5~5.0 eV) is facilitated by band gap offsets. Therefore, hole concentration in MWNT is remarkably decreased by electron-hole recombination, which affects hole transport through the nanotube network and thus leads to the increase in Iph. Results indicate that an electrostatic assembly of PbSCys/MWNT nanocomposites shows an excellent photoresponse and good cycling performance due to favorable charge transfer at the interface. This QD/CNT nanostructure can be developed for high-performance photoswitches by optimizing the electrostatic interaction between QDs and nanotubes.

Conclusion

Well-crystallized PbSCys QDs after the phase transfer from toluene to water were uncovalently attached onto PD-MWNT by the electrostatic assembly. The packing density of PbSCys was controlled by tuning the weight ratios of QDs to MWNT. PbSCys/MWNT nanocomposites displayed an improved NIR absorption because of the presence of uniform mono-dispersion of PbS QDs on the sidewalls. Compared with PbSCys, a remarkable quenching of PL (28.6%) and a blue-shifted (300 nm) band of PbSCys/MWNT indicated the efficient charge transfer between PbS and MWNT at the interface. This result was further investigated by a two-terminal device based on PbS/MWNT nanostructures. Under light illumination, photo-induced electron transfer from PbS to MWNT resulted in an excellent on/off switching photocurrent and good stability during 20 cycles. PbSCys/MWNT5:1 nanocomposite showed a high photo sensitivity up to 31.3% with the photocurrent of 18.3 μA under the light of 3.85 mW/cm2, which outperformed many QD/carbon-based nanocomposites. This work paves a way for the fabrication of high-performance optoelectronic devices using the electrostatic layered assembly of PbS/MWNT nanostructures.

Methods

Materials

Raw MWNT was purified by the mild oxidation in H2O2 solution41 and treated in the mixture of sulfuric acids and nitric acids to generate many oxygen-groups on the sidewalls. Water-dispersed MWNT was obtained after rinsing with distilled water for several times by filtration until pH of filtrate is 7. Lead (II) acetate trihydrate (Pb(OAc)2), OA and thioacetamide (Alfa Aesar) were used as received and Cys, Tri-n-octylphosphine (TOP), tetramethylammonium hydroxide pentahydrate (TMAH) and PDDA (20%, w/w, in water, Mw = 200000–350000) were purchased from Sigma-Aldrich. All other reagents were used without further purification.

Synthesis of OA-capped PbS QDs

PbSOA QDs were synthesized by an organometallic chemistry method42. Typically, Pb(OAc)2 (0.91 g, 2.4 mmol), OA (2.8 mL, 8.2 mmol), diphenylether (8 mL, 50.3 mmol) and distilled TOP (4 mL, 9.0 mmol) were mixed at room temperature. The mixture was purged by nitrogen and heated at 90°C for 1 h. Subsequently, the sulphur-precursor solution of thioacetamide (0.64 g, 0.88 mmol) and TOP (4.8 mL, 10.8 mmol) in 0.4 mL of N,N-Dimethylformamide was added to the above mixture at 90°C. The reaction was carried out for 12 h at 100°C. The resultant products were washed with isopropyl alcohol by centrifugation to remove residual precursors. PbSOA QDs were dispersed in toluene to form a uniform solution (3 mg/mL).

Synthesis of aqueous Cys-capped PbS QDs by the phase transfer

The phase transfer of PbSOA QDs into a water solution was carried out by the ligand exchange43. Cys (121.16 mg, 1 mmol) and TMAH (360 mg, 2 mmol) were dissolved in 10 mL of methanol to form a CTMA salt as a phase transfer agent. PbSOA QDs toluene solution (0.5 mL) was added to 5 mL of chloroform. While stirring, 0.5 mL of CTMA solution was added dropwise to the PbSOA QDs solution and ultrapure water (5 mL) was also added as an aqueous solvent at a reduced stirring speed. QDs were transferred to the upper aqueous phase after a few minutes and further purified with acetone by centrifugation and re-dispersed in ultrapure water. The resulting aqueous PbSCys QDs (Yield: 80%) were obtained.

Synthesis of PbSCys/MWNT by the electrostatic self-assembly

Water-dispersed MWNT (5.0 mg) was dispersed in 200 mL of water by ultrasonication for 1 h. PDDA (0.5 mL) as a cationic PE, 0.5 M NaCl and 1.0 M NaOH were mixed in uniform MWNT dispersion by ultrasonication for 5 h. MWNT was encapsulated by PDDA by the electrostatic interaction. PD-MWNT with positive charges on the surface and good stability in an aqueous solution (0.5 mg/mL) was acquired after removing excessive PDDA by washing with di-water for several times via centrifugation. PbSCys/MWNT nanocomposite with a layered nanostructure was prepared by the electrostatic self-assembly at room temperature as shown in Figure 1. Typically, 5 mL of negative charged PbSCys QDs (0.1 mg/mL) solution was added to a certain volume (1 mL or 5 mL) of positively charged PD-MWNT solution at a concentration of 0.1 mg/mL (Table 1). The weight ratios of PbSCys to MWNT were 5:1 and 1:1 for PbSCys/MWNT5:1 and PbSCys/MWNT1:1, respectively. The total volume of the solution was 10 mL by adding ultrapure water. The mixture was ultrasonicated for 10 min and the electrostatic interaction was carried out for 30 min under stirring. The resultant layered assembly of PbSCys/MWNT hybrid was gained after removing excess QDs by subsequent centrifugation and rinse with di-water for several times. Of particular were the PL measurements, which were performed on unpurified hybrid samples at the same concentration of QDs.

Fabrication of the device

A two-terminal device (photoswitch) was fabricated as follows: An n-type Si wafer with a 450 nm thick layer of thermally grown SiO2 dielectric was used as the substrate. Conventional photolithography process was performed to fabricate the microelectrode plate by patterning Au contact pads on top of the substrates. Si/SiO2 substrate was ultrasonically rinsed in distilled water, alcohol and acetone successively and dried with a nitrogen gun. PbSCys/MWNT nanocomposite was dispersed in di-water to form a uniform solution. This hybrid solution was drop-casted on the surface of SiO2. The device was dried and stored in vacuum.

Characterization

The microstructures of PbS QDs and PbSCys/MWNT hybrid were observed by HRTEM (Tecnai G2 F20). FT-IR spectroscopy was carried out on a Bruker Tensor 27 spectrometer to analyze chemical structures. UV-vis-NIR absorption and PL were studied by a spectrophotometer (UV-3600, Shimadzu) and a spectroflurometer (FL3-221-TCSPC, HORIBA Jobin Yvon Inc), respectively. Morphologies of PbSCys/MWNT hybrids on the substrate of the photoswitch were observed by field-emission scanning electron microscopy (FESEM, Hitachi S-4800).

The device was irradiated by a 150 W halogen lamp (Beijing Changtuo Technology Company). This light source delivered a continuous spectrum ranging from 200 to 2000 nm. The incident white light was focused and guided by a long optical fiber to avoid the heating effect and the power intensity delivered on the photoswitch was measured using the light density meter (Beijing Zhongjiaojinyuan Co., Ltd.). Photo-switching properties upon light irradiation with different power densities were recorded by keithley 4200-SCS and a micromanipulator 6150 probe station in a clean and shielded box at room temperature in air.