Energy band modification for UV photoresponse improvement in a ZnO microrod-quantum dot structure

Abstract

In this paper, ZnO quantum dots (QDs) were decorated on an individual ZnO microrod surface to construct a UV photodetector with good performance. The perfect hexagonal ZnO microrod creates a totally internal reflective configuration for enhanced light–matter interaction while the quantum confinement of the QDs extends the energy bandgap for broader spectral response. More importantly, the QD modified microrod constructs a favorable charge transfer system for effective UV photoresponse. As the ZnO QDs were decorated on the microrod, the photocurrent and light-to-dark current ratio were increased by about 2 magnitude orders while the response and recovery speed were accelerated by more than 1 order of magnitude. The material preparation, device fabrication, optoelectronic behavior and mechanism are investigated in detail. This research presents a facile and viable approach to design optoelectronic devices by integrating quantum dots into the same bulk semiconductor.

---

Introduction

Ultraviolet (UV) photodetectors¹ have been paid considerable attention in recent years because of their potential applications in flame sensing, missile warning, and environmental monitoring.^2,3 An ideal photodetector for such applications generally allows the device to have a high on/off current ratio, fast response and recovery speed, good linear photocurrent response to the incident optical power, and high spectral selectivity.⁴ Wide band gap semiconductors, including ZnO, ZnS, SiC and GaN etc., have been employed to perform UV detection.⁵ Among these alternatives, ZnO is an ideal candidate for visible-blind UV-light sensors owing to its suitable band gap (≈3.37 eV), variable synthetic strategies,⁶ compatible alloy process and high carrier mobility.⁷ So far, ZnO-based UV photodetectors have been fabricated by using single crystals, thin-films and low-dimensional nanostructures.^8–11 Compared with the traditional thin-film devices, nanostructure-based UV photodetectors usually present higher responsivity and photoconductivity gain because of their large surface-to-volume ratio and the reduced dimensionality of the active area.¹² Especially, the one-dimensional (1D) ZnO microrod with high crystalline quality and high carrier mobility provides direct conduction pathway for effective charge transport. Park et al.¹³ reported a high electron mobility of above 1000 cm² V⁻¹ s⁻¹ in field-effect transistors (FETs) of MOCVD-grown ZnO nanorods, and Chang et al.¹⁴ obtained the mobility over 4000 cm² V⁻¹ s⁻¹ from the CVD-grown ZnO nanowires after surface passivation. Meanwhile, the perfect hexagonal crossection of ZnO microrod can strongly confine the light inside and enhance the light–matter interaction based on the whispering gallery mode microcavity effect.¹⁵ So the ZnO microrod photodetector is expected to exhibit higher photocurrent gain and faster response speed than the conventional thin-film and bulk structure devices. On the other hand, the enlarged band gap of ZnO QDs due to the quantum confinement effect¹⁶ would extend the photocurrent response to visible-blind even sunlight-blind band, while the large absorption coefficient would result in stronger absorption to improve UV detection. Additionally, an abundant of “particle–particle” junction barriers in ZnO QDs network are also favorable to enhance photoresponse under the UV irradiation.¹⁷

In this paper, ZnO QDs were integrated into an individual ZnO microrod to form a hybrid UV detector, which exhibits superior photoresponse characteristics including high sensitivity, fast response speed and broad UV range photoresponse. The perfect hexagonal ZnO microrod creates a totally internal reflective configuration for enhancing light–matter interaction and provides an excellent transport path for photogenerated charge carriers. Furthermore, the decorated quantum dot processes an inherent nature of a large absorption coefficient above its band gap, while the quantum confinement of the QDs extends the energy bandgap for broader spectral response. More importantly, the ZnO microrod/QDs hybrid structure creates an efficient charge transfer^18,19 for superior photoresponse through the energy band modification.

Experimental section

ZnO microrods were fabricated in a horizontal tube furnace through a vapor phase transport route. A mixture of ZnO and graphite powders with mass ratio of 1 : 1 was filled into a small quartz boat as source materials, and a Si substrate was covered on the boat with the polished surface facing down. The microrods were obtained on the substrate after reaction for 40 minutes at 1150 °C. ZnO QDs were prepared through a sol–gel route similar to the previous reports.²⁰ The as-synthesized ZnO QDs were purified by repeated centrifugation and re-dispersed in absolute ethanol. To characterize the ultraviolet photoresponse properties, two Ag electrodes with gap of about 50 μm were deposited on a clean quartz substrate by radio frequency magnetic sputtering. An individual ZnO microrod was picked up from the Si substrates and transferred across the electrode gap. Afterwards, the ZnO QDs were deposited by drop casting on the single ZnO microrod device. In order to improve the interface contact between ZnO microrod and QDs, after the drop casting, the samples were baked at 80 °C for 30 minutes to evaporate the deposited chemical solvent.

The morphology of as-prepared products were characterized by field emission scanning electron microscopy (FE-SEM, Carl Zeiss Ultra Plus) and transmission electron microscopy (TEM, JEM-2100). The absorption measurement was carried out by a UV-vis spectrophotometer (Cary 5000, Varian). The current–voltage (I–V) characteristics and photocurrent of the photodetectors were measured by Keithley 4200 using a two-probe method. The photocurrent was measured when the device is illuminated by a 325 nm laser. The optical power illuminated on the devices was adjusted with neutral density filter and monitored with an energy meter. A spectral response for different wavelengths was recorded by using a xenon lamp (power density = 24 mW cm⁻²). All of the measurements were carried out at room temperature in ambient condition.

Results and discussion

Fig. 1a shows a SEM image of a typical ZnO microrod with a hexagonal cross section and smooth facets, which is beneficial to form a good contact with electrodes. More importantly, the perfect hexagonal cross-section of the ZnO microrod constructs a whispering gallery microcavity to confine strongly the light inside and enhance the interaction between the light and the ZnO material based on the totally internal reflective.¹⁵ The inserted SEM image reveals uniform size of about 6 μm in diameter and 2 mm in length of the ZnO microrods grown vertically on the Si substrate. A TEM image of the ZnO QDs in Fig. 1b shows the uniform size with an average diameter of 3 nm. The inserted high-resolution TEM image clearly displays the lattice fringes with 0.26 nm d-spacings, which corresponds to the (0001) planes of wurtzite ZnO.

Fig. 1 (a) SEM image of an individual ZnO microrod of perfect hexagonal cross section and smooth side facets. Inset: SEM image of vertical ZnO microrods array on the silicon substrate. (b) Low-magnification TEM image of the ZnO QDs. Inset: High-resolution TEM image of same ZnO QD showing lattice fringes.

In order to investigate the effect of ZnO QDs on the optical properties of ZnO microrods, the UV-vis optical absorption spectra have been carried out at room temperature. Fig. 2 shows the optical absorption spectra of bare ZnO microrods and QDs-decorated microrods. The absorption spectra of ZnO QDs in ethanol solution is also illustrated in the inset for reference which shows the absorption peak at 330 nm. The absorption of ZnO microrods dramatically enhances as the wavelength is shorter than 370 nm. The absorption spectrum of the ZnO microrods/QDs hybrid almost load two parts absorption from both of them with two peaks at 330 nm and 370 nm. Furthermore, the QDs-decorated microrods show considerable enhancement in the absorbance throughout the ultraviolet light region, which is attributed to the combination of the efficient absorption from both ZnO QDs and microrods. The result indicates that the ZnO microrods/QDs hybrid structure broadens the absorption spectrum and improves the absorbency for UV light compared to the bare ZnO microrods.

Fig. 2 UV-vis optical absorption spectra of ZnO microrod before and after ZnO QDs decoration. The absorption spectra of ZnO QDs in ethanol solution is also illustrated in the inset for reference.

The SEM image in Fig. 3a exhibits the photodetector configuration of both ZnO microrod with and without ZnO QDs. The ZnO QDs are too small to be observed in this magnification. Fig. 3b and c show typical I–V characteristics of the samples measured using a two-probe method in dark and under UV light illumination (λ = 325 nm, power density = 300 mW cm⁻²), respectively. The nonlinearly increased current vs. the applied voltage indicates a Schottky barrier at the interfaces between the ZnO microrod and the Ag electrodes. It is clearly seen that the dark currents of the two samples are all in the magnitude of 10⁻⁹ A at an applied voltage of 5.0 V, while the photocurrent of QDs-decorated ZnO microrod is significantly higher than that of the bare one. The increase in photocurrent of the ZnO microrod/QDs hybrid-based photodetector should be attributed to the both contribution of the ZnO microrod and a large number of ZnO QDs.^21,22 Due to the quantum confinement effect, the broadened conduction and valence bands of the ZnO QDs nest on those of microrods to construct a favorable energy level configuration for carrier charge transport.^23,24 The proposed photoresponse mechanism of the ZnO microrod/QDs hybrid structure is shown in Fig. 3d. The electron–hole pairs generates in both ZnO microrod and QDs under the illumination of UV light. The photogenerated carriers in QDs will transfer to the coupled microrod due to the internal electric field at the microrod/QD interface, leading to a high concentration of carriers in ZnO microrod. As a large amount of electrons moving into the microrod, the high carrier mobility in the high-crystalline microrod makes it an effective conduction channel for conducting electrons. Therefore, the additional photogenerated electrons in QDs and the excellent conduction path along the axis of microrod contribute to the increase of the photocurrent.

Fig. 3 (a) A representative SEM image of the photodetector fabricated from the ZnO microrod. I–V characteristics of ZnO microrod photodetector (b) without ZnO QDs decoration and (c) with ZnO QDs decoration both in dark and under 325 nm UV light illumination. (d) Schematic diagram of the photoresponse mechanism of ZnO microrod/QDs composite.

To examine the repeatability and response speed of the ZnO UV detectors, the time-resolved photocurrent at 5 V bias with on/off cycles of multiple 325 nm UV light has been measured. Both turn-on and turn-off time of UV light are 30 s. As shown in Fig. 4a and b, the photocurrent increased steeply to a stable value as soon as the light was switched on, and then dramatically decreased to its initial value as the light was turned off. No degradation was found in multiple on–off switching cycles, confirming the good stability and reproducibility of our devices. The rise time (τ_r) and the decay time (τ_d) were measured to evaluate the speed of response and recovery processes, which are defined as the time rise to (1 − 1/e) of the maximum photocurrent from dark current and recover to 1/e of the maximum photocurrent, respectively. It is worth noting that both the response and recovery speed of ZnO UV detector decorated with ZnO QDs is faster than the bare one. According to the rising and falling edges shown in Fig. 4c and d, the calculated τ_r and τ_d are 2.02 s and 4.66 s for the bare ZnO microrod, and 0.25 s and 0.44 s for the QDs-decorated ZnO, respectively. The following two important factors are taken account for the fast response speed although the reason for this remarkable enhancement is not quite clear yet. As shown in Fig. 3d, the energy band modification in ZnO microrod/QDs hybrid will enhance the band-bending at interface and further accelerate the separation of the electron–hole pairs. When the UV light is turned on or off, the photogenerated electrons and holes in the interface region change quickly, leading to a fast transient response.^25,26 On the other hand, the chemi-adsorption and photo-desorption of oxygen molecules on the surface of ZnO microwire are responsible for their photoresponse.²⁷ In dark, oxygen molecules adsorb on the ZnO microwire surface by capturing free electrons from the n-type ZnO [O₂(g) + e⁻ → O₂⁻(ad)], thereby creating a depletion layer with low conductivity near the surface. Upon exposure to UV-light, photo-generated holes migrate to the surface and discharge the adsorbed oxygen ions through surface electron–hole recombination [O²⁻(ad) + h⁺ → O₂(g)]. These unpaired electrons transport to the external circuit under the applied bias to produce current. But the chemi-adsorption and photo-desorption of oxygen molecules are relatively slow process, which result in a slow response speed. In our device, the ZnO QDs may provide partial passivation and minimize the interaction of oxygen with ZnO surface.^28–30 Thus the UV response of device was dominated by the photogenerated electrons and holes, which transferred quickly under the illumination of UV light in the interface region.

Fig. 4 (a and b) The reproducible on/off switching of the devices upon 325 nm light illumination with a ∼30 s cycle at a bias of 5.0 V. (c and d) Experimental and fitting curves of photocurrent rising and falling processes of the devices.

The time-resolved photocurrent response under different UV illumination powers is given in Fig. 5. It can be seen that the photocurrent increases significantly with rising of the illumination power. The photogenerated carrier density could affect the Fermi energy and the work function of photosensitive semiconductors, while simultaneously adjust the contact-potential barrier height and width semiconductor/electrode interface.¹⁵ Therefore, the photocurrent responds sensitively to the illumination intensity based on the nonlinear dynamic mechanism. The dependence of photocurrent on the light intensity was plotted logarithmically and inserted in Fig. 5a and b. The photocurrent I_light vs. illumination power P curves can be fitted well with a simply power law:

I_light = AP^χ

where A is a constant for certain wavelength and exponent χ determines the response of photocurrent to the light intensity. In our measurements, χ is 0.34 and 0.55 for the ZnO microrod before and after QDs decoration, respectively. The non-unity exponent is thought to be related to the complex process of photocarrier generation, trapping, recombination, and carrier transport during the photoresponse process.³¹ The differences of the increased exponent χ between the two samples demonstrate the existence of the carriers transfer between the ZnO microrod and the QDs.

Fig. 5 Photocurrent of the ZnO microrod (a) without QDs decoration and (b) with QDs decoration in response to different power of 325 nm light illumination. The inset shows the illumination power-dependent photocurrent at 5 V bias. The curves are fitted well by a power law.

Fig. 6a and b depict the photon-response spectra of the ZnO microrod photodetector before and after ZnO QDs decoration as a function of the incident light wavelength at a bias of 5.0 V. The responsivity is very low when the illumination wavelength is longer than 400 nm, and dramatically increases as soon as the illuminated photon energy is above the bandgap of ZnO which is in good agreement with the UV-vis optical absorption spectra. The high spectral selectivity suggests that the present photodetector is ‘visible-blind’ and highly UV-sensitive. The responsivity of the QDs-decorated UV detector increases over 10 times comparing with that of the bare one. The improvement of the responsivity is closely related to the efficient charge transfer between QDs and microrod. The photogenerated electron–hole pairs can be excited in both ZnO microrod and QDs under the UV light illumination. The QDs modify the energy band of the ZnO microrod, then the photogenerated carriers in QDs will transfer to the coupled microrod efficiently. As a large amount of electrons moving into the microrod, the high carrier mobility in the high-crystalline microrod provides an excellent conduction path for the electrons to the collection electrode, and improves the responsivity. Besides increased responsivity, the QD-decorated ZnO microrod also displays a broader response in the wavelength range between 300 to 400 nm. Comparing the two photoresponse curves in Fig. 6, it indicates that the increased photocurrent in shorter wavelength region between 300 and 350 nm is mainly ascribed to the photogenerated electron–hole pairs from the ZnO QDs. Because the quantum confinement of the QDs extends the energy bandgap for broader spectral response.

Fig. 6 Spectral responsivity of the ZnO microrod photodetector (a) without and (b) with ZnO QDs decoration measured at a constant bias of 5.0 V.

Conclusion

In summary, ZnO QDs decoration was carried out as an effective way to enhance both photocurrent and response speed of single ZnO microrod photodetector. The fabricated ZnO microrod/QDs hybrid shows obvious enhancement in absorption throughout the ultraviolet light region at room-temperature. The photocurrent and light-to-dark currents ratio increased about 2 magnitude orders as the ZnO QDs were decorated on the microrod while the both response and recovery time became faster more than 1 magnitude order. Several key factors play important roles simultaneously to achieve the obtained results. First, the perfect hexagonal crossection of ZnO microrod creates a totally internal reflective microcavity for improved UV response. Furthermore, ZnO microrod provides an excellent conduction path for photogenerated charge carriers while the quantum confinement of the QDs extends the energy bandgap for broader spectral response. More importantly, the efficient charge transfer can be easily achieved due to the energy band modification between the QD and ZnO microrod. This provides an approach to design optoelectronic devices with high efficiency based on the same semiconductor material in bulk and quantum states.

Acknowledgements

This work was supported by the project of the National Natural Science Foundation of China (11304127), the Jiangsu University Foundation for Qualified Personnel (1283000262) and funding from the State Key Laboratory of Bioelectronics of Southeast University (1721220150).

Notes and references

1. H. Y. Chen, K. W. Liu, L. F. Hu, A. A. Al-Ghamdi and X. S. Fang, Mater. Today, 2015, 18, 493–502.
2. E. Monroy, F. Omnès and F. Calle, Semicond. Sci. Technol., 2003, 18, 33–51.
3. X. Gong, M. Tong, Y. Xia, W. Cai, J. S. Moon, Y. Cao, G. Yu, C.-L. Shieh, B. Nilsson and A. J. Heeger, Science, 2009, 325, 1665.
4. A. A. Chaaya, M. Bechelany, S. Balme and P. Miele, J. Mater. Chem. A, 2014, 2, 20650–20658.
5. L. Peng, L. F. Hu and X. S. Fang, Adv. Mater., 2013, 25, 5321–5328.
6. X. H. Huang, C. B. Tay, Z. Y. Zhan, C. Zhang, L. X. Zheng, T. Venkatesan and S. J. Chua, CrystEngComm, 2011, 13, 7032–7036.
7. X. Y. Ji, T. Wang, L. L. Guo, J. Q. Xiao, Z. Y. Li, L. M. Zhang, Y. Deng and N. Y. He, J. Biomed. Nanotechnol., 2013, 9, 417–423.
8. H. S. Bae, M. H. Yoon, J. H. Kim and S. Im, Appl. Phys. Lett., 2003, 83(25), 5313–5315.
9. H. Kind, H. Q. Yan, B. Messer, M. Law and P. D. Yang, Adv. Mater., 2002, 14, 158–160.
10. J. F. Lu, C. X. Xu, J. Dai, J. T. Li, Y. Y. Wang, Y. Lin and P. L. Li, Nanoscale, 2015, 7, 3396.
11. M. Chen, L. F. Hu, J. X. Xu, M. Y. Liao, L. M. Wu and X. S. Fang, Small, 2011, 7(17), 2449–2453.
12. S. Liu, J. F. Ye, Y. Cao, Q. Shen, Z. F. Liu, L. M. Qi and X. F. Guo, Small, 2009, 5, 2371.
13. W. I. Park, J. S. Kim, G.-C. Yi, M. H. Bae and H. J. Lee, Appl. Phys. Lett., 2004, 85(21), 5052.
14. P.-C. Chang, Z. Fan and C.-J. Chien, et al., Appl. Phys. Lett., 2006, 89, 133113.
15. J. Dai, C. X. Xu, X. Y. Xu, J. Y. Guo, J. T. Li, G. Y. Zhu and Y. Lin, ACS Appl. Mater. Interfaces, 2013, 5, 9344–9348.
16. X. Y. Xu, C. X. Xu, Z. L. Shi, C. Yang, B. Yu and J. G. Hu, J. Appl. Phys., 2012, 111, 083521.
17. X. Y. Xu, C. X. Xu and J. G. Hu, J. Appl. Phys., 2014, 116, 103105.
18. B. Zhao, F. Wang, H. Y. Chen, Y. P. Wang, M. M. Jiang, X. S. Fang and D. X. Zhao, Nano Lett., 2015, 15, 3988–3993.
19. B. J. Yang, J. T. Chen, L. F. Cui and W. W. Liu, RSC Adv., 2015, 5, 59204–59207.
20. X. Y. Xu, C. X. Xu, X. M. Wang, Y. Lin, J. Dai and J. G. Hu, CrystEngComm, 2013, 15, 977–980.
21. H. Seong, J. Yun, J. H. Jun, K. Cho and S. Kim, Nanotechnology, 2009, 20, 245201.
22. W. J. Zheng, X. C. Li, G. H. He, X. M. Yan, R. Zhao and C. X. Dong, RSC Adv., 2014, 4, 21340–21346.
23. K.-F. Lin, H.-M. Cheng, H.-C. Hsu, L.-J. Lin and W.-F. Hsieh, Chem. Phys. Lett., 2005, 409, 208–211.
24. X. Y. Xu, C. X. Xu, Z. L. Shi, C. Yang, B. Yu and J. G. Hu, J. Appl. Phys., 2012, 111, 083521.
25. M.-W. Chen, C. Y. Chen, D.-H. Lien, Y. Ding and J.-H. He, Opt. Express, 2010, 18(14), 14837–14841.
26. Y. F. Hu, J. Zhou, P.-H. Yeh, Z. Li, T.-Y. Wei and Z. L. Wang, Adv. Mater., 2010, 22, 3327–3332.
27. R. Debnath, T. Xie, B. M. Wen, W. Li, J. Y. Ha, N. F. Sullivan, N. V. Nguyend and A. Motayedaf, RSC Adv., 2015, 5, 14646–14652.
28. B. Mallampati, S. V. Nair, H. E. Ruda and U. Philipose, J. Nanopart. Res., 2015, 17, 176.
29. J. Zhou, Y. D. Gu, Y. F. Hu, W. J. Mai, P.-H. Yeh, G. Bao, A. K. Sood, D. L. Polla and Z. L. Wang, Appl. Phys. Lett., 2009, 94, 191103.
30. R. S. Aga Jr, D. Jowhar, A. Ueda, Z. Pan, W. E. Collins, R. Mu, K. D. Singer and J. Shen, Appl. Phys. Lett., 2007, 91, 232108.
31. C. S. Lao, M.-C. Park, Q. Kuang, Y. Deng, A. K. Sood, D. L. Polla and Z. L. Wang, J. Am. Chem. Soc., 2007, 129, 12096–12097.

---