Quantum Dots-Facilitated Printing of ZnO Nanostructure

Jun 20, 2017 - Department of Energy's National Security Campus, Kansas City, Missouri 64147, United States. ACS Appl. Mater. Interfaces , 2017, 9 (27)...
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Quantum Dots-Facilitated Printing of ZnO Nanostructure Photodetectors with Improved Performance Brent Cook,*,† Qingfeng Liu,*,† Maogang Gong,† Dan Ewing,‡ Matthew Casper,‡ Alex Stramel,‡ and Judy Wu*,† †

Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States Department of Energy’s National Security Campus, Kansas City, Missouri 64147, United States

‡

S Supporting Information *

ABSTRACT: A nanocomposite ink composed of zinc oxide precursor (ZnOPr) and crystalline ZnO quantum dots (ZnOPrQDs) has been explored for printing high-performance ultraviolet (UV) photodetectors. The performance of the devices has been compared with their counterparts’ printed from ZnOPr ink without ZnO QDs. Remarkably, higher UV photoresponsivity of 383.6 A/W and the on/off ratio of 2470 are observed in the former, which are significantly better than 14.7 A/W and 949 in the latter. The improved performance is attributed to the increased viscosity in the nanocomposite ink to enable a nanoporous structure with improved crystallinity and surface-to-volume ratio. This is key to enhanced surface electron-depletion effect for higher UV responsivity and on/off ratio. In addition, the QD-assisted printing provides a simple and robust method for printing high-performance optoelectronics and sensors. KEYWORDS: inkjet printing, nanocomposite ink, quantum dots, nanoporous zinc oxide, photodetector

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INTRODUCTION For the past few decades, technological progress has followed the empirical Moore’s law of miniaturization of microelectronics.1 This has led to an increasing interest in research and development of nanomaterials such as carbon nanotubes,2,3 semiconductor quantum dots4−6 and nanostructures,7−9 twodimensional graphene,10−12 and transition metal dichalcogenides13−15 for next-generation electronics and optoelectronics. While exciting progress has been made in demonstration of the extraordinary performance in the devices based on the nanomaterials, integration of such devices with the industrial standard complementary metal oxide semiconductors (CMOS) remains a challenge due to the incompatibility of the CMOS fabrication with the approaches employed for devices based on nanomaterials. Ultrasonic inkjet printing offers a promising and costeffective method for integrating devices based on nanomaterials with CMOS with advantages of position-specific deposition, low material waste, and scalability.16,17 To achieve a wellcontrolled ink dispersion in terms of ink-drop morphology, thickness, and dimension, the prepared ink must have the right Reynolds, Weber, and Ohnesorge numbers. These quantities characterize the ink and depend on the ink density, travel velocity, viscosity, and surface tension.18,19 Among these three numbers, the Ohnesorge number NO is the most important. NO is the ratio of the Weber number NW and Reynolds number NR and is defined by NO =

NW NR

=

η γρα

the ink viscosity, surface tension, density, and characteristic length, respectively.18,19 The inverse of the Ohnesorge number is the Z parameter and is an indicator of droplet formation. For inkjet printing a value of 1 < Z < 10 is preferred to avoid too high viscosity (Z < 1) and too high fluidity (Z > 10), respectively.18,19 In practical printing, the selection of the inks may depend on the specific material under consideration. Taking ZnO as an example, a direct bandgap semiconductor with a bandgap (∼3.4 eV) in the ultraviolet (UV) spectrum, and a large exciton binding energy (60 meV),20 has been employed for wide applications such as gas sensors,21,22 acoustic sensors,23,24 UV photodetector, and so on.25−27 Typically, the aim of the inkjet printing of ZnO is to create transparent thin films for fieldeffect transistors, which is often riddled with nonuniform rough films with poor crystallinity.28−30 Many inks have been experimented with for ZnO printing including zinc acetate/ methoxyethanol/ethanolamine29,30 and zinc nitrate/ammonia hydroxide mixtures,31,32 while most of them produce rough and porous films with nonuniform feature size and thickness on the order of nanometers to micrometers. It should be noted that the microporous structure of the ZnO films is disadvantageous to most applications due to a low film density. This differs from Received: April 16, 2017 Accepted: June 20, 2017 Published: June 20, 2017

, where η, γ, ρ, and α are

© XXXX American Chemical Society

A

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ZnOPrQDs ink was made with ZnO QDs, the ZnO QD fabrication process is described in detail in previous work.38 The ZnO QDs were mixed with 0.3 M Zn(AC)2 precursor to make the ZnOPrQD ink; the concentration of ZnO QDs is about 0.2 mg/mL in the solution. A SonoPlot Microplotter with a glass capillary attached to an ultrasonicating piezoelectric device for dispensing was used for sample printing. Interdigitated electrodes with channel length of 100 μm were fabricated using standard photolithography followed with e-beam deposition of Au (40 nm)/Ti (5 nm) on SiO2 (500 nm)/Si wafer before printing. A single layer of the selected ink was printed on interdigitated electrodes. The printer is an ultrasonic inkjet printer that functions by dispensing the ink through a glass capillary attached to a piezoelectric. The dispensing was controlled by the voltage acting on the piezoelectric, which determines the strength of the ultrasonication. Depending on the amount of fluid present in the capillary the dispensing voltage can vary, however not by much, for both inks dispensing voltage varied from 1 to 2 V. Printed samples were then annealed in a furnace at 350 °C for 2 h in air before characterization of the optoelectronic properties. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were employed using a FEI Tecnai F20 XT field emission transmission electron microscope to characterize the crystallinity and dimension of the ZnO QDs, which showed high crystallinity and narrow diameter range of 5−6 nm. Scanning electron microscopy (SEM) images were taken with a JEOL JSM-6380 SEM system to analyze the sample morphology. For optoelectronic characterization, a Newport Oriel Apex monochromator with an Oriel Cornerstone 130 1/8m monochromator filter was used.

nanoporous ZnO morphology when the feature size is comparable to the Debye length (∼19 nm) to allow an optimal advantage of the surface electron-depletion effect (Figure 1).33 In contrast to the bulk ZnO in which electron carrier concentration is unaffected by the surface (Figure 1a), the electron concentration in ZnO nanostructures can be strongly reduced by orders of magnitude due the surface electrondepletion effect caused by localization of the electrons by oxygen attached to the ZnO surface. An optimal electrondepletion effect is therefore anticipated when the ZnO feature size is comparable to the Debye length with a large surface-tovolume ratio (Figure 1b). The reduced dark current and enhanced photocurrent result in improved on/off ratio and photoresponsivity upon UV illumination.34 The Z value for aqueous solutions composed of methoxyethanol, ethanolamine, and water can range from 10 to 30, which makes control of the ZnO film morphology into nanoporous structure difficult.35−37 In order to resolve this critical issue, we explore a novel nanocomposite ink of zinc acetate mixed with ZnO quantum dots (ZnO QDs) for printing nanoporous ZnO UV detectors in this work. In addition to this nanocomposite ZnOPrQD ink, a regular ZnO acetate precursor (ZnOPr) was also employed for the comparison purpose. The incorporation of the highly crystalline ZnO QDs is expected to generate two effects. First, the ZnO QDs of diameter of 5−6 nm could generate a scaffold to the otherwise highly fluidic zinc acetate ink, promoting the nanoporous morphology to form and preventing surface tension driven microporous morphology. On the other hand, the high crystallinity of the ZnO QDs can serve as highly crystalline seeds for ZnO nucleation from zinc acetate, reducing the growth defects and impurities. Remarkably, higher photoresponsivity by more than an order of magnitude has been observed in ZnO UV detectors printed from ZnOPrQDs ink as opposed to that from the reference ZnOPr ink. This large difference in the device performance can be well-explained by the difference in the nanoporous morphology in the former, as compared to the microporous one in the latter. Specifically, the printed ZnOPrQDs UV detectors with channel length 100 μm show a responsivity up to 383.6 A/W at 5 V bias and rise and fall times of 16 and 14 s, respectively, with a film thickness from 100 to 200 nm, which is comparable in the responsivity to other costlier high-quality ZnO films (∼400 A/W at 5 V) with rise and fall times of 1.0 and 1.5 μs, film thickness of 1.0 μm, and channel length of 2.0− 16 μm produced by chemical vapor deposition;25 RF sputtered ZnO films (124 A/W) with rise and fall times of 0.82 and 0.64 ms, film thickness of 200 nm, and channel length of 25 μm;26 and spin-coated precursor for ZnO nanoparticles (0.1 A/W) with rise and fall times of 20 and 350 ns, film thickness of 1.0 μm, and channel length of 5.0 μm.27 The lower response time in nanostructured ZnO is associated with the surface oxygen desorption/adsorption in response to light on/off, while this surface electron-depletion effect contributes to the photoconductive gain and therefore enhancement of the photoresponsivity in ZnO nanostructured ZnO optoelectronic devices. This result therefore illustrates the viability of nanocomposite inks for the printing high-performance ZnO nanostructure devices.

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RESULTS AND DISCUSSION Figure 1a is a drawing of bulk ZnO; the free electrons in bulk ZnO are from natural defects in the crystal and have been

Figure 1. Schematic description of the effect of surface electron depletion that is (a) negligible in bulk ZnO, (b) while significant in nanostructured ZnO. In the absence of UV light, free electrons on the ZnO surface are captured by oxygen attached to the surface creating a depletion region of depth on the order of the Debye length. These electrons are delocalized upon UV illumination and therefore contribute to the photoresponse.

noted as the cause for the n-type behavior of ZnO. The Debye length of ZnO (λ ∼ 15−20 nm)39,40 is the thickness of the surface layer of ZnO at which the electrons are depleted (or localized) by the adsorbed oxygen at the surface of ZnO (e− + O2(g) → O2−(ad)). In bulk ZnO this surface effect is negligible due to the small surface-to-volume ratio, so bulk properties dominate. In ZnO nanostructures, such as ZnO nanoparticles shown in Figure 1b with substantially larger surface-to-volume ratio, the contribution of the surface layer increases dramatically. When the radius of the ZnO nanoparticles becomes comparable to the Debye length, the entire volume of the nanoparticle is affected by the surface electronic depletion effect, resulting in reduction of the dark current (or conductivity) by orders of magnitude from that in the n-type ZnO bulks. When UV light excites an electron from the valence band to the conduction band of ZnO, a hole is formed and

EXPERIMENTAL SECTION

The ZnOPr ink was made of zinc acetate dihydrate (0.3 M Zn(AC)2· 2H2O) precursor solution ultrasonicated for 10 min or longer with 0.3 M ethanolamine and 2-methoxyethanol to make a clear sol−gel.34 The B

DOI: 10.1021/acsami.7b05324 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces migrates toward the surface, resulting in the release of oxygen, h+ + O2−(ad) → O2(g) and therefore delocalization of the electron. The process increases the charge concentration (photoconductive gain) and results in increased photocurrent and, hence, the photoresponsivity in ZnO nanostructures. It should be noted that this enhanced photoresponsivity is at the cost of slower photoresponse41,42 (or longer response time), since the surface electron-depletion effect is associated with the slower surface oxygen adsorption/desorption process as compared to the much faster interband photoexcitation in ZnO bulk.33,43 However, the much enhanced photoresponse in ZnO nanostructures makes them appealing for many practical applications which demand high sensitivity. Figure 2 illustrates schematically the two ZnO nanostructure photoconductive photodetectors printed using the ZnOPr

Figure 3. SEM images of (a, b) a single layer ZnO films printed from the ZnOPr ink at different magnifications respectively and (c, d) a single layer of ZnO films printed from the ZnOPrQDs ink at the scales to panels a and b, respectively.

pores have irregular shapes with dimensions in the range of sub-micrometer to a few micrometers. It is understandable that the formation of the pores may closely relate to the surface of the substrates. The hydrophobic surface of the SiO2/Si substrates used in this work plays a critical role in forming the microporous ZnO films printed from the ZnOPr ink due to the large contact angle of the ink on the substrate surface. In contrast, the addition of the ZnO QDs into the Zn(AC)2 precursor ink led to considerably denser and smoother nanoporous ZnO films except some ridges and valleys (Figure 3c,d). In fact, no large-dimension pores are visible, which indicates the presence of the ZnO QDs alters the contact angle of the ZnOPrQDs ink with the substrates to promote the even spread of the ink on the substrate surface. The sample morphology and crystallinity were further investigated using TEM, and the results of the samples printed respectively from the ZnOPr and ZnOPrQDs inks are compared in Figure 4. Interestingly and consistently with the SEM result in Figure 3, the former exhibits a considerably larger grain size (Figure 4a) as compared to the latter case (Figure 4c). This indicates ZnO QDs indeed provide a control over the morphology and structure of the printed ZnO film, especially to obtain a nanoporous ZnO structure for optimal surface electron-depletion effect. It should be pointed out that the feature size of the sample printed from the ZnOPrQDs ink is comparable to the Debye length, suggesting the ZnO QDs of 5−6 nm in dimension (about one-third of the Debye length) are effective in controlling the feature size in the desired range. This argument is supported by the higher photoresponse observed in the nanoporous films printed from the ZnOPrQDs ink as compared to their microporous ZnOPr counterparts to be discussed in the following text. In addition, the samples printed from different inks have different crystallinities and microstructures as shown in Figure 4b,d. While ZnO nanocrystallites are clearly visible in both cases, the dimension of the crystallites in the ZnOPr sample (Figure 4b) is considerably larger than that of the ZnOPrQDs sample (Figure 4d). In particular, ZnO QDs are clearly visible in the latter case (red circles in Figure 4d). This means the incorporated ZnO QDs play a critical role in facilitating a more uniform nucleation of ZnO nanocrystallites by providing a large amount of interfaces between the highly crystalline ZnO QDs and Zn(AC)2 precursor. These interfaces can provide lower energy

Figure 2. Schematic of inkjet printing with two types of inks of (a) ZnOPr and (b) ZnOPrQDs. (c) TEM images of ZnO QDs and higher resolution TEM of a QD (inset) and (d) ZnO QD size distribution extracted from TEM analysis.

(Figure 2a) and ZnOPrQDs (Figure 2b) inks, respectively. The ZnO QDs have a uniform dimension of 5−6 nm, which is much smaller than the Debye length (Figure 2c,d). The small dimension of the ZnO QDs is favorable to realize the optimal surface electron-depletion effect. It is also important to maintain the ZnO QDs well-dispersed in the ZnOPr ink in order to obtain the nanoporous ZnO structure. High crystallinity can be clearly seen with a lattice spacing of 0.26 nm expected for ZnO along the (0001) direction (inset of Figure 2c). When mixed in the ZnOPr precursor ink, the ZnO QDs were well-dispersed in the ZnOPrQDs ink before printing and remained uniformly distributed in the ink droplets after printing but sometimes would gather at the edges of ink puddles upon drying. The morphology of the two types of samples printed respectively with the ZnOPr and ZnOPrQDs inks differs considerably as shown in the SEM images (Figure 3). Without the ZnO QDs in the Zn(AC)2 precursor ink, the samples have a highly rough and porous morphology (Figure 3a,b). The C

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Figure 5. (a) Dark current and illuminated (UV light intensity of 0.52 mW/cm2) current as a function of the bias voltage of ZnO films printed using ZnOPr (black) and ZnOPrQDs (red) inks, (b) spectral UV responsivity under a UV power of 5.84 μW as a function of voltage, (c) photoresponsivity divided by the maximum responsivity, and (d) ratio of the photoresponsivity of ZnOPrQDs with respect to ZnOPr.

Figure 4. (a, c) TEM and (b, d) HRTEM images of a single layer of printed ZnO using (a, b) ZnOPr and (c, d) ZnOPrQDs inks.

nucleation sites for ZnO.43−45 In addition, the more uniform and higher concentration of nucleation sites lead to smaller ZnO nanoparticles (NPs) interconnected into a favorable nanoporous network as shown in Figure 4c. However, ZnO QDs as seeds for growth of larger ZnO NPs may not be the predominant case as most crystallites have isotropic shapes and many show dimension comparable to that of the ZnO QDs. In addition, Figure S1a (Supporting Information) compares the absorption spectra of the ZnOPr and ZnOPrQDs samples printed on fused silica. It can be seen that the absorption spectra of the ZnOPrQDs and ZnOPr samples are comparable. In both samples, a strong UV absorption band edge at ∼365 nm can be observed, corresponding to the intrinsic bandgap of ZnO at ∼3.4 eV. In addition, the absorption in the visible region is negligible. Moreover, the photoluminescence (PL) was measured at room temperature on the ZnOPr and ZnOPrQDs samples (Figure S1b, Supporting Information). Both samples exhibit a strong UV emission at ∼385 nm, which is expected as the characteristic band-edge emission of crystalline ZnO.46,47 Compared to the ZnOPrQDs sample, a low and broad visible emission that is associated with the defects in crystalline ZnO was detectable in the ZnOPr sample. This demonstrates that ZnO QDs enable a nanoporous structure with improved crystallinity and surface-to-volume ratio, as suggested by the SEM and TEM results. The current−voltage characteristics of the printed ZnOPr and ZnOPrQDs samples were measured in dark and under a monochromatic light source with a wavelength of 340 nm at UV power of P = 10.45 μW, and UV intensity I = 0.52 mW/ cm2. The wavelength of 340 nm was chosen because it has a photon energy of ∼3.6 eV, which is just above the energy bandgap of ZnO at ∼3.4 eV, which is approximately wavelengths ∼ 365 nm, respectively. The results are compared in Figure 5a. The ZnOPr and ZnOPrQDs detectors have similar dark currents (Idark) while the latter has significantly higher illuminated current (IUV). This may be attributed to the nanoporous morphology of the ZnOPrQDs sample with higher surface-to-volume ratio and therefore higher electron-depletion

effect, as compared to the microporous morphology in the ZnOPr sample. The on/off ratio is defined as the ratio of photocurrent Iph = IUV − Idark and Idark from Iratio =

Iph Idark

of 2470

in the latter is considerably higher than that of 949 in the Iph

former. The photoresponsivity defined from R = P at 5.0 V are 14.7 and 383.6 A/W respectively for the of ZnOPr and ZnOPrQDs samples. The improved performance by more than an order of magnitude in the ZnOPrQDs sample as compared to the ZnOPr one illustrates the importance of controlling the morphology and microstructure of the printed ZnO nanostructure UV detectors. The nanoporous structure obtained using ZnO QD-assisted printing from the ZnOPrQDs ink indicates the nanocomposite inks by inclusion of ZnO QDs may provide a versatile approach toward such a control. Figure 5b exhibits the responsivity as a function of voltage measured on both ZnOPrQDs and ZnOPr samples showing a linear growth in photoresponsivity at increasing voltage. The ratio of photoresponsivity at 325 nm with respect to the responsivity at other wavelengths shows a band edge around 375 nm is clearly visible across which the responsivity increases (Figure 5c). In addition, the normalized photoresponsivity of ZnOPr depicts a photoresponsivity that is larger than ZnOPrQDs film at 375 nm, which indicates that the band edge for ZnOPr is broadened, allowing lower photon energies to excite electrons into the conduction band. The band-edge broadening is likely caused by poorer crystallinity in the ZnOPr film, whereas ZnOPrQDs shows a band edge closer to highly crystalline ZnO. The observed trend in spectral responsivity is consistent with high crystallinity of the printed ZnO nanostructure samples revealed from the TEM measurement in Figure 4. The larger variation of about 4 orders of magnitude across the band edge in the former, as compared to about 3 orders of magnitude variation in the latter, may be attributed to the better crystallinity with ZnO QDs addition in the former. The responsivity ratio of ZnOPrQDs to ZnOPr as a function of D

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the printed ZnO, which is promising to integrating highperformance nanostructure devices with CMOS in practical applications.

UV intensity at the 1.0 V bias is shown in Figure 5d; the responsivity ratio shows ZnOPrQDs are approximately 6 times the responsivity of ZnOPr at lower intensities. The responsivity ratio shows little change from higher and lower intensities of UV light, indicating the charges generated are proportional to the available photons; this means ZnOPrQDs film generates more charge per photon at all intensities than does the ZnOPr film. In Figure 6, the dynamic response is compared on the two representative samples printed from the ZnOPr and

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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/acsami.7b05324. Absorption and PL spectra of the printed ZnOPr and ZnOPrQDs samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.C.). *E-mail: qfl[email protected] (Q.L.). *E-mail: [email protected] (J.W.). ORCID

Brent Cook: 0000-0001-9288-7267 Qingfeng Liu: 0000-0003-2492-8092 Notes

Figure 6. Dynamic response. (a) ZnOPrQDs has rise and fall times of 55 and 84 s, and (b) ZnOPr show rise and fall times 16 and 14 s, respectively. Both films were tested at a 5 V bias.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Plant Directed Research and Development funds from the Department of Energy’s National Security Campus, operated and managed by Honeywell Federal Manufacturing and Technologies, LLC under Contract No. DE-NA0002839. J.W. acknowledges support in part by ARO Contract No. ARO-W911NF-16-1-0029 and NSF Contract Nos. NSF-DMR-1337737 and NSF-DMR-1508494.

ZnOPrQDs inks. The former shows rise and fall times of 16 and 14 s, while, for the latter, 55 and 84 s. These rise/fall times on the order of a few to a few tens of seconds are among the best reported for ZnO nanostructure photodetectors, which can be attributed to the high crystallinity of ZnO nanostructures with low defect concentration as charge traps. However, this photoresponse is much slower than that of highly crystalline bulk ZnO based on the photoexcited interband transition33,34,41−43 due to the large time frame needed for oxygen desorption (upon UV illumination) and re-adsorption (UV off) on the ZnO nanostructure surface. This means the significantly enhanced photoresponsivity due to the contribution of the surface electron-depletion effect in the nanostructured ZnO UV detectors is at a cost of the reduced response speed. This also explains the moderately increased response times in the ZnOPrQDs sample due to the increased surface-to-volume ratio in these nanoporous ZnO films printed with the assistance of the ZnO QDs.

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REFERENCES

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CONCLUSION In summary, this work has explored quantum dots-facilitated printing of ZnO nanostructure photodetectors through development of a nanocomposite ZnOPrQD ink consisting of highly crystalline ZnO QDs (prefabricated) with zinc acetate precursor. The ZnO QDs of 5−6 nm in diameter were found effective in controlling the microstructure of the printed ZnO films. In contrast to the microporous structure in the inkjetprinted ZnO films using the ZnOPr ink, a nanoporous structure was obtained using the nanocomposite ZnOPrQD ink under a comparable inkjet printing condition. Importantly, the feature size in the obtained nanoporous ZnO is close to the Debye length of ZnO (∼20 nm) desired for an optimal surface electron-depletion effect. Consequently, much enhanced UV responsivity up to 383.6 A/W at 5.0 V bias was obtained on the printed nanoporous ZnO detectors, which is more than an order of magnitude higher than that in the microporous ZnO counterparts. This result demonstrates the approach of the nanocomposite ink is viable in controlling the microstructure of E

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DOI: 10.1021/acsami.7b05324 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX