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Abstract
Quantum dots (QDs) are exploited in visible light communication (VLC) due to their unique optical properties. However, it is still a challenge to conquer heating generation and photobleaching under prolonged illumination. In this paper, we proposed to utilize hexagonal boron nitride (h-BN) nanoplates to improve the thermal stability and photo stability of QDs and long-distance VLC data rate. After heating to 373 K and cooling to the initial temperature, photoluminescence (PL) emission intensity recovers to 62% of the original intensity and after 33 hours of illumination, PL emission intensity still maintains 80% of the initial intensity, while that of the bare QDs is only 34% and 53%, respectively. The QDs/h-BN composites perform a maximum achievable data rate of 98 Mbit/s by applying on-off keying (OOK) modulation, while the bare QDs are only 78 Mbps. In the process of extending the transmission distance from 0.3 m to 5 m, the QDs/h-BN composites exhibit superior luminosity corresponding to higher transmission data rates than bare QDs. Particularly, when the transmission distance reaches 5 m, the QDs/h-BN composites still show a clear eye diagram at a transmission rate of 50 Mbps while the eye diagram of bare QDs is indistinguishable at 25 Mbps. During 50 hours of continuous illumination, the QDs/h-BN composites keep a relatively stable bit error rate (BER) at 80 Mbps while that of QDs continuously increase, and the -3 dB bandwidth of QDs/h-BN composites keep around10 MHz while the bare QDs decrease from 12.6 MHz to 8.5 MHz. After illumination, the QDs/h-BN composites still indicate a clear eye diagram at a data rate of 50 Mbps while that of pure QDs is indistinguishable. Our results provide a feasible solution for realizing an enhanced transmission performance of QDs in longer-distance VLC.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, visible light communication (VLC) has attracted significant attention. Compared to radio frequency (RF) communication, VLC performs well in spatial flexibility in the unregulated band, which is ∼1300 times more than the bandwidth available in the RF spectrum [1–3]. Moreover, VLC is unrestricted in several special scenarios, such as underwater communication [4–6], high-precision positioning and navigation [7–9], and non-line-of-sight free-space optical communication [10–12]. One of the most popular scenes is integrating the transmitter with the existing lighting, or directly color-converting its output which can offer both in-door lighting and high-speed communication. However, the color converters in the conventional transmitter of the VLC system are mostly commercial phosphors with long photoluminescence (PL) lifetime, which limits the maximum data rate of the VLC system to up to 10 Mbit/s [13]. Although some techniques such as optical blue-filtering, pre-equalization, and decision feedback equalization have been implemented to enhance the transmission performance, the complexity of optical design and the extra modulation circuit also increases the complexity of system design and restrict the implementation of the VLC system [14,15].
Alternative efforts have been carried out to solve the above problem by replacing commercial phosphor such as yttrium aluminum garnet (YAG) [16] phosphor with quantum dots (QDs) [17], perovskite nanocrystals [18–21] and organic dyes [22]. For example, CdSe/ZnS QDs combined with LED were experimentally demonstrated by Xiao et al. with a -3 dB bandwidth of 2.7 MHz [23,24]. Zhao et al. used Cs3Cu2Cl5@SiO2 nanocrystals in a VLC system with orthogonal frequency division multiplexing modulation with a bit loading to get a data rate of 2.65 Mbps [25]. Mo et al. synthesized CsPbBr3@ZrO2 with a -3 dB bandwidth of 2.7 MHz and a data rate of 33.5Mbps [26]. However, the nanocrystals combined with LED or LD often faced thermal damage caused by the excessively concentrated energy distribution [27]. In most cases, synthesizing composite structures by integrating thermal conductive materials is a typical solution to improve the thermal and light stability of fluorescent materials [28]. Compared to several thermal conductive materials, such as graphene [29], carbon nanotubes [30], and silicon dioxide (SiO2) [31], hexagonal boron nitride (h-BN) nanoplates are excellent reinforcing fillers with their negligible light absorbance within the visible range and ultrahigh thermal conductivity (≈300 W m−1 K−1) along the 2 D structure [32], which are promising solutions for improving the thermal stability of nanocrystals in VLC system.
In this paper, we proposed to improve the long-distance VLC data rate by using h-BN nanoplates to encapsulate CdZnSeS/ZnSeS QDs. The QDs are uniformly dispersed in h-BN nanoplates without aggregation and their quantum yield (QY) can be high as 67%. Due to the high intrinsic thermal conductivity, chemical stability of h-BN, and passivation of surface states for QDs, the thermal and photon stability of QDs/h-BN hybrid structures are greatly improved compared to bare QDs. Finally, the as-prepared QDs/h-BN composites are employed with an LD as a signal transmitter in VLC systems. The data rate decreases slowly as the distance lengthened and a clear eye diagram can be observed when the data rate is 50 Mbps at a long distance of 5 m. The BER and -3 dB bandwidth remain relatively stable during long time illumination and show a clear eye diagram at 50 Mbps after illumination.
2. Materials and optical properties
The colloidal QDs referred to in this work were synthesized based on our recent report [33]. The fabrication procedure of the QDs/h-BN composites is shown in Fig. 1. First, h-BN powder was mixed with toluene at a concentration of 25 mg/mL. After 10 hours of ultrasonic vibration, the h-BN were dispersed homogeneously and the solution became clear. Then, 40 μL of the above products were mixed with 100 μL QDs solution and stirred for 10 min. Finally, the QDs/h-BN composites were obtained and stored in the dark for further experiments.
The morphology of the h-BN nanoplate is characterized as a sheet-like structure with a lateral dimension of several hundreds of nanometers and a thickness of less than 100 nm, as shown in Fig. 2(a). Such a high specific surface area and abundant mesopores provide sufficient sites for the adsorption of the QDs nanocrystals [28]. Figure 2(b) shows the SEM image of QDs/h-BN nanocomposites which indicates the introduction of QD does not change the morphology of the h-BN. Figure 2(c) shows the energy-dispersive spectrometer (EDS) mapping of the QDs/h-BN composite, which reveals a successful integration and uniform distribution of QDs. Considering the relatively small size of QDs, the QDs/h-BN composite was further measured with TEM as shown in Fig. 2(d). which further proves that the QDs are uniformly dispersed on the surface of h-BN nanoplates without aggregation.
Fig. 2. SEM images of (a) h-BN and (b) QDs/h-BN composites. (c) SEM element mapping images showing the elemental distribution of B, N, and Cd. (d) TEM image of QDs/h-BN composites.
Figure 3(a) shows the PL emission spectra of as-prepared QDs/h-BN composites and bare QDs. The peak wavelength and full-width-at-half-maximum (FWHM) of QDs were measured as 609 nm and 32.4 nm, respectively. After being incorporated with h-BN nanoplates, QD’s peak wavelength and FWHM changed to 610 nm and 32.3 nm, which indicates the amount of additional h-BN nanoplates barely influences the emission peak position since the QDs are not aggregated. In contrast, the QY of QDs/h-BN composite increases from 52% to 67% compared to pure QDs, resulting from the passivation of the surface defects of the QDs. The fluorescence dynamics of QDs and QDs/h-BN were measured with the time-correlated single photon counting (TCSPC) method, as shown in Fig. 3(b). Both decay curves can be well fitted by the biexponential function:
Table 1. PL decay time fitting parameters of CdZnSeS/ZnSeS QDs and CdZnSeS/ZnSeS QDs/h-BN composites.
Temperature-dependent experiments were performed to evaluate the thermal stability of the as-prepared composites, as shown in Fig. 4(a). When the temperature rises from 303 K to 373 K and then recovers to 303 K, the PL intensity of the pure QDs is only 34% of the original state. In contrast, the PL intensity of synthesized QDs/h-BN composites recover 62% after the above experiment, which results from the excellent heat conductor of h-BN. The results indicate that the thermal stabilities of the as-prepared composites are greatly improved after introducing the h-BN since the uniform distribution of QDs on the 2D h-BN nanoplates mitigates the degradation and aggregation of nanocrystals under thermal stress [30]. To simulate the light and heat environment, 2 W UV light continuously irradiates samples for 33 hours, as shown in Fig. 4(b). It is apparent that except for the high thermal stability, QDs/h-BN composite possesses excellent photo-stability, which still exceeds 80% of the pristine intensity after 33 h under continuous UV exposure compared to pure QDs which decreased to 53%. In addition, QDs/h-BN composites exhibit an increase in the PL intensity during the first 6 hours of illumination, which may be due to surface modification of h-BN. The photo-activated rearrangement of surfactant molecules can passivate the surface trap states, thereby increasing the probability of thermalization back to the lowest emitting exciton state and enhancing the PL property [36]. Furthermore, the hygroscopic properties of h-BN can absorb water molecule on the QD’s surface which can induce the photo-passivation of the charge carrier traps states [37].
Fig. 4. (a) Thermal stability of QDs and QDs/h-BN composites. (b) Photostability of QDs and QDs/h-BN composites under continuous illumination.
3. Communication performance
The schematic diagram of the experimental setup for QD-LD based VLC system using an OOK modulation scheme is illustrated in Fig. 5. The transmitter consists of a 405 nm LD and red-emitting CdZnSeS/ZnSeS QDs or QDs/h-BN composites. In the experiments, an arbitrary waveform generator (AWG, Tektronix, 5200) with a maximum sampling rate of 5 GSa/s was used to generate the modulated signal. Through a bias-Tee (Mini-circuit, ZFBT-6GW+,0.1 MHz-6000 MHz) circuit, the modulated signal was biased by a direct current signal to generate the positive signal, which was used to drive the LD with a designed circuit board. The mixed light from LD-based QDs-converter was collimated by the transmitter lens (Tx, lens), and then transmitted through the air with a distance of 0.3 m,1 m,2 m,3 m, and 5 m, respectively. The transmitted emission light was detected by a high-sensitivity avalanche photodiode (Thorlabs, APD210,1 MHz-1600 MHz) through a receiver lens (Rx lens) and a long pass edge filter (Semrock, 473 nm). The electric signal was captured to obtain the frequency response by network analyzer (Keysight, 100 KHz-3 GHz) or was offline demodulated and analyzed with a MATLAB program for calculating bit error rate (BER) by a digital storage oscilloscope (OSC, Tektronix, MSO46). To ensure the consistency of the excitation light signal throughout the experiment, the excitation from the LD was fixed with a current of 50 mA.
Fig. 5. Experimental setup of the LD-based QDs-converter VLC system. AWG: arbitrary waveforms generator; OSC: oscilloscopes; PC: Personal computer; APD: avalanche photodiode.
Figure 6(a) shows the emission spectra of LD under different bias currents from 34 mA to 82 mA measured by a spectrometer (Princeton Instruments, Acton SP2750), which indicates that UV LD can perform FWHM less than 1 nm with a slow red-shift trend of the central wavelength with the increase of drive current resulted from the thermal effect. Figure 6(b) shows the optical power - current-voltage (P - I - V) of the LD, indicating a turn-on voltage of 3 V and a lasing threshold at 40 mA. The peak output power of ∼ 16 mW was achieved at 80 mA with a bias of 5 V. The modulation bandwidth characteristics of the LD were explored and plotted in Fig. 6 (c), where the narrow line around 400 nm indicates attenuated light from the LD. As shown in Fig. 6(c), the -3 dB frequency response of the device was measured to be 416 MHz, 824 MHz, 832 MHz, 936 MHz, and 1056 M Hz at 25 mA, 44 mA, 60 mA, 80 mA, and 98 mA, respectively. The natural frequency response of the LD shows a continuous flat response without the equalization circuit, which is within the response bandwidth of the detector. Figure 7(a) shows the PL spectrum of LD-based QDs-converter working at 5 V. The received optical power before the Rx lens without further focus is shown in Fig. 7(b), the received optical power drops inevitably resulted from PL emission scattering as the transmission distance increases from 0.3 m to 5 m, where the luminous intensity of the QDs/h-BN composites is still exhibited to be higher than that of pure QDs. Compared to the pure QDs, the presence of a certain amount of h-BN was proven to enhance the emission performance of QDs, which can be attributed to the scattering effect of h-BN that increase the probability of laser-QDs interactions, according to Mie theory [38]. According to a previous study, the relationship between the -3 dB response bandwidth and decay time of fluorescent materials is analogous to the response bandwidth and carrier lifetime in electroluminescent devices [39]:
where $\tau $ is the differential lifetime of carriers. However, when applied to fluorescent materials, the lifetime that directly attenuates the luminous intensity to 1/e is quite different from the fitting effect of formula (2). Considering that the decay curves of our samples are fitted with a biexponential function, we speculate that the band-edge luminescence and defect-trapping luminescence of QDs correspond to different frequency responses, respectively. In fact, by substituting the surface defects luminescence lifetime of QDs (12.3 ns) and QDs/h-BN composite (14.57 ns) into formula (2), the -3 dB response bandwidth is generally consistent with the measured 12.9 MHz and 10.9 MHz, as shown in Fig. 7(c).Fig. 6. LD operating performance. (a) Laser spectra of with different current, (b)P-I-V curve. (c)Frequency response.
Fig. 7. (a) PL spectra of QD-based LD. (b) Comparison of the received optical power under the conditions with pristine QDs and QDs/h-BN composite at different transmission distances. (c) Frequency response of QD and QD/h-BN composite.
To further demonstrate the QDs/h-BN composites color converter is superior to the pure QDs in free-space VLC, communication performance tests at different transmission distances were performed. We first determined the maximum transmission data rate for pure QDs and QDs/h-BN composites, as shown in Fig. 9(a), when the optical transmission distance is 0.3 m. It can be observed that the BER is 0.0041 for pure QDs at a data rate of 80 Mbps, which exceeds the FEC threshold (3.8 × 10−3), while that for composites at a data rate of 96 Mbps exactly reach 0.0038, indicating that the introduction of h-BN nanoplates have an improvement on communication performance for QDs. When the transmission distances are lengthened from 1 m to 3 m, the QDs/h-BN composites maintain much higher data rates than the pure QDs, as shown in Fig. 8(b)-(d). When the distance increases from 3 m to 5 m, the captured eye diagrams at data rates of 10 Mbps, 12.5 Mbps, and 25 Mbps for QDs and 25Mbps, 50Mbps, and 60Mbps for QDs/h-BN composites, as shown in Fig. 9(a)-(f), respectively. It is apparent that compared to a not distinguishable eye diagram at 25 Mbps for pure QDs, QDs/h-BN composites maintain a clear eye diagram at the same data rate, even at 50 Mbps. The distance of 5 m is perfectly adequate for the transmission distance for watching TV and lighting, which satisfies people’s activity in indoor areas.
Fig. 8. Bit error rate variety of CdZnSeS/ZnSeS QDs and CdZnSeS/ZnSeS QDs/h-BN composites at the detection distance of 0.3 m,1 m,2 m, 3 m.
Fig. 9. Eye diagrams of CdZnSeS/ZnSeS QDs (a)-(c) from 10 Mbps to 25 Mbps and CdZnSeS/ZnSeS QDs/h-BN composite (d)-(f) from 25 Mbps to 60 Mbps over 5-m transmission link.
To verify the addition of h-BN nanoplates on long-time communication stability enhancement of QDs, we assessed the variation of communication performance for pure QDs and QDs/h-BN composites under continuous-wave laser at an optical power of 5 mW for more than 50 hours. As shown in Fig. 10(a), the BER of composites remained relatively stable for the first 35 hours, after which that shows an increasing tendency due to prolonged illumination damage. And the BER of composites remains below the FEC threshold for 50 hours, while that of pure QDs is over the FEC limit and shows an increasing tendency. There is a decreasing trend of BER for composite within the first 6 hours, which is consistent with the photoenchancement results above. The -3 dB bandwidth of the composites remains stable for the first 35 hours and tends to decrease after that, while that of the pure QDs decreases from 12.6 MHz to 8.5 MHz, further indicating the enhancement of the stability of the QDs communication performance by h-BN nanoplates, as shown in Fig. 10 (b). Figure 10(c)-(d) shows a comparison of the eye diagram for pure QDs and QDs/h-BN composite at a data rate of 50 Mbps after 50 hours of continuous illumination. It is clear to observe that the eye diagram of pure QDs is not distinguishable at 50 Mbps while that of composites is still relatively clear.
Fig. 10. CdZnSeS/ZnSeS QDs and CdZnSeS/ZnSeS QDs/h-BN composites under continuous illumination for 50 hours (a) BER variation (b) -3 dB bandwidth variation (c) eye diagram comparison.
4. Conclusion
In summary, we proposed to introduce h-BN as a heat-conducting medium to the QDs to prepare QDs/h-BN composites and utilize those composites in the VLC system. The h-BN can improve the photoluminescence property of QDs by surface passivation. The QDs/h-BN composites manifest excellent thermal stability and photostability. The PL emission intensity of QDs/h-BN composites decrease to 17% and recovers to 62% of the initial temperature compared to pure QDs. After 33 hours of continuous illumination, PL emission intensity still maintains 80% of the initial intensity. In the 33 hours photostability experiment, the QDs/h-BN composites still maintained a PL emission intensity of 80% even underling heating. Finally, the as-prepared QDs/h-BN composites are employed with an LD as a signal transmitter in a VLC system. Since the scattering effect of h-BN can increase the probability of laser-QDs interactions, QDs/h-BN composite exhibits higher emission intensity, and improved signal-to-noise of VLC system, showing better communication performance of keeping 50 Mbps data rate at the long distance of 5 m. The enhancement of photostability from h-BN keep QDs stable BER and -3 dB bandwidth during long-time illumination, showing a clear eye diagram after illumination at 50 Mbps. These results prove a promising implementation of QDs/h-BN composites in VLC applications in the future.
Funding
Zhejiang Province Public Welfare Technology Application Research Project (LGN22F050002); National Natural Science Foundation of China (62205084).
Acknowledgments
We thank Westlake Center for Micro/Nano Fabrication for the facility support and technical assistance. We thank Instrumentation and Service Center for Molecular Sciences at Westlake University for PL spectral and delay curves measurement.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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