Ultrahigh photosensitivity of the polar surfaces of single crystalline ZnO nanoplates

Hyun Woo Noh a, Soon Moon Jeong b, Junghyun Cho ac and Jung-Il Hong *ad
aDepartment of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea. E-mail: jihong@dgist.ac.kr
bSmart Textile Convergence Research Group, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea
cDepartment of Mechanical Engineering & Materials Science and Engineering Program, State University of New York (SUNY) at Binghamton, Binghamton, New York 13902, USA
dResearch Center for Emerging Materials, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea

Received 21st January 2018 , Accepted 3rd March 2018

First published on 5th March 2018


Single crystalline ZnO nanoplatelet structures were synthesized via a hydrothermal process on the surface of GaN microparticles. Growth of ZnO seeded on the GaN surface promoted faster growth along the directions within the basal plane of the ZnO crystal structure, resulting in the formation of 2-dimensional nanoplates with a thickness less than a few tens of nanometers at most. Electrical conduction across an individual nanoplate was measured and found to be extremely sensitive to UV illumination and the surrounding atmospheric environment. Such electrical behaviors of the nanoplates were attributed to the dominance of the polar (0001) surfaces and the adsorption and desorption of the ambient gas molecules on these surfaces. Their coupling with conduction electrons near the surface is the critical factor responsible for the highly sensitive electrical properties of the nanoplate. Virtually the entire volume of the nanoplates is under the influence of the surface adsorbed molecules, which changes the electrical properties of the nanoplates extensively, depending on their environmental conditions. Combining the very high photocurrent to dark current ratio and the high effective resistance of the ZnO nanoplates reported in the present study, ultrasensitive photo-devices operating at very low power can be expected with the use of 2-dimensional nanoplates.


During the last three decades, there has been growing interest towards zinc oxide (ZnO) nanostructures because it is relatively easy with ZnO to achieve various nanoscale morphologies and structures, including nanowires and nanoparticles. Furthermore, with the availability of simple and low-cost processing in hydrothermal synthesis, ZnO is considered as one of the best test frames to study as well as to test modern nanotechnology based on nanoscale materials.1–4 With the consideration of these inherent characteristics, ZnO has been quite extensively studied in the fields of optoelectronics, sensors, and transducers, utilizing its multifunctional properties as a wide band gap semiconductor and piezoelectric oxide originating from its crystallographic asymmetries.5–7 Various forms of ZnO nanostructures such as nanocombs, nanorings, nanohelixes/nano-springs, nanobelts, nanowires, and nanocages have been demonstrated with solid–vapor thermal sublimation techniques.8–10 However, such a process to fabricate nanostructures has limitations with regard to its high cost and high temperature requirement. Thus, significant efforts have consequently been directed toward overcoming these issues. On the other hand, the hydrothermal synthesis method has been considered as one of the attractive approaches because of its easy fabrication under simple autogenous reactor conditions at low temperatures, and it has been highly utilized to grow 1-dimensional nanowire structures.11–15 Recently, growth of 2-dimensional nanoplate structures by the hydrothermal method has also been reported to be possible with the use of heterogeneous seed layers,16,17 and the polyvinylpyrrolidone (PVP) surfactant18 for ZnO and Bi2Te3 nanoplates, respectively, and their properties were found to be distinctly different from those found in nanoparticles of various shapes19,20 or nanowire structures.

In the present study, we fabricated free-standing ZnO nanoplates and nanorods to investigate and compare the photo-electrical properties that vary depending on their crystallographic facets or morphologies. Electrical currents were flown perpendicular and parallel to the c-axis of the hexagonal ZnO structure in the nanoplates and nanorods, respectively, for comparison purpose, and the anisotropic properties of nanostructures were explored. Tests were carried out under various light illumination conditions at several different excitation wavelengths (from 254 nm to 395 nm). The nanoplate device exhibited a remarkably high UV on/off photocurrent ratio with a photocurrent gain of up to 2 × 105 under UV 254 nm illumination compared to the nanowire device. Based on the measurement results, we attempted to understand and control the formation of conduction channels in the surface-dominating structures of ZnO nanoplates. With these results, we also explore the possibility of utilizing ZnO nanoplates for photosensitive devices.

For the simple chemical synthesis of ZnO nanostructures, zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMTA) purchased from Sigma-Aldrich were dissolved completely in deionized (DI) water to form a 10 mM solution of each. The two solutions were then mixed and stirred at room temperature to make a homogeneous solution.21 GaN powder particles were dispersed and attached on a soda-lime glass substrate using double-sided polyimide tape and were introduced to be placed vertically in the middle of the solution. This mixture solution was then kept in a sealed reactor inside an oven at 80 °C for 3 hours. After the solution was cooled down at room temperature, the sample was washed with isopropyl alcohol (IPA) and dried with a nitrogen gas flow. In this procedure, GaN particles were found to play a decisive role in controlling the growth direction of ZnO nanostructures. With the identical crystal symmetries of ZnO and GaN, the surface of GaN serves as a good nucleation site for the growth of ZnO nanostructures in solution.8,10 However, the lattice mismatches along the c- and a- axes are different between the two phases, and the growth of ZnO on the surface of GaN was found to be predominantly along the a-axis direction along which lattice mismatch is lower (<0.4%), thereby the synthesized nanostructures are mostly in the shape of hexagonal nanoplates (Fig. 1(a)). Identical synthesis procedures were used for the growth of ZnO nanorods except for the substrates on which nanostructures grew. Using the same solutions, the Si(001) substrate was located on the surface of the mixture solution floated by surface tension. Then the solution was maintained at 80 °C in an oven for over 3 days. Long hexagonal ZnO rods grew on the surface of the silicon substrate, as in Fig. 1(b), dispersed in random directions without any specific orientational relationship with the substrate.10


image file: c8nr00569a-f1.tif
Fig. 1 Scanning electron microscopy (SEM) images show (a) ZnO nanoplates and (b) nanorods. White bars in (a) and (b) represent the length of 5 μm. SEM images of an individual nanoplate and a nanorod with attached electrodes for the electrical characterization are also shown in the insets. The distance between the electrodes are also 5 μm. XRD patterns for the corresponding phases are in (c) and (d).

The crystal morphologies of the grown structures were examined using a Hitachi S-4800 field emission scanning electron microscope (FE-SEM). X-ray diffraction (XRD) was also performed using a PANalytical X-pert Pro diffractometer with Cu Kα radiation to verify the crystal structures and orientations, as shown in Fig. 1(c) and (d) for ZnO nanoplates grown on GaN powder and ZnO nanorods grown on the Si substrate, respectively. All diffraction peaks matched well with the Wurtzite structures of ZnO and GaN as found in the power diffraction database. Relatively stronger diffraction peaks at 31.78°, 34.43°, and 36.26° were identified as the (100), (002), and (101) peaks. It is noted that the (100) and (101) peaks for ZnO and GaN are indistinguishable between the two phases, while the (002) peaks are clearly resolvable for the two phases.22 Hexagonal crystallite shapes of the plates observed in the SEM micrograph (Fig. 1(a)) are also consistent with the XRD patterns. The width of nanoplates was in the range of 5 to 10 μm and the thickness varied from 10 to 40 nm, which gives the width to thickness ratio of a few hundreds (ranging from 125 to over 500). From the comparison with the report by Li et al.17 about the chemical growth of hexagonal ZnO nanoplates on the Al surface with the width to thickness ratios of ∼32, it can be said that the nucleation of ZnO on the GaN surface shows higher efficiency for the controlled growth of ZnO nanoplates due to the low lattice mismatch between them.

In order to directly measure the electrical characteristics of the synthesized nanoplates and nanorods, Au electrode patterns were fabricated on the Si substrate with a 300 nm thick SiO2 insulating layer on the top surface by conventional photolithography patterning procedures. An Au electrode layer of 100 nm thickness was deposited by the radio frequency (RF) magnetron sputtering technique. Individual nanoplate or nanorod structures were dry transferred to the substrate with electrode patterns as carried out by Dai et al.23 Connection of the nanoplates to the existing electrode pads was completed by the electron-beam lithography process with a distance of 5 μm yielding the structures (indicated as ‘s’ and ‘d’) shown in the inset of Fig. 1(a), where a device made from a broken piece of a hexagonal nanoplate is shown. On the other hand, for the fabrication of the ZnO nanorod device, the length of a nanorod was long enough to make electrical contact with the patterned Au electrodes without the additional e-beam lithography process. Electrical contacts for a nanorod (also labelled as ‘s’ and ‘d’) are shown in the inset of Fig. 1(b). Then, IV and photocurrent characteristics of the fabricated devices were measured using a Keithley semiconductor characterization system, SCS-4200, with and without ultraviolet (UV) illumination. UV illumination was applied using commercial light emitting diodes (LEDs) of specific output wavelengths (λ = 265, 285, 310, 365, 385, and 395 nm). The light intensity was adjusted by the distance between the LED light source and the device under test. Fig. 2(a) and (b) show the measured IV results of ZnO nanoplates (a) and nanorods (b), respectively, under dark conditions and under UV illumination of 0.5 mW cm−2 light intensity. The measurements were also carried out under different environmental conditions of vacuum and a nitrogen atmosphere as well as at an elevated temperature (∼380 K) to evaluate the environmental effects on the ZnO nanoplate interconnects under the UV light of 254 and 365 nm wavelengths at 0.15 mW cm−2.


image file: c8nr00569a-f2.tif
Fig. 2 IV relationships were measured from individual (a) nanoplates and (b) nanorods under UV illumination (0.5 mW cm−2) of various wavelengths as indicated. Photocurrent to dark current ratios for (c) nanoplates and (d) nanorods were measured to vary with opposite trends.

It is notable that the electrical resistivity of the nanoplate was found to be significantly greater than that of ZnO nanorods. Upon the application of voltage across the width of the ZnO nanoplates via the deposited electrodes from 0 to 10 V, as shown in the inset of Fig. 1, a nearly negligible amount of current (0.0432 pA) was measured to flow (see Fig. 2(a)), which is in contrast to the conduction of a few μA in a nanorod under the same applied voltage of a few volts. With the consideration of the width and thickness of the nanoplates and nanorods through which conduction currents flow, the resistivity of the nanoplate is estimated to be on the order of 107 Ω cm, which is ∼106 times greater than that of the typical nanorod in the range of a few tens of Ω cm. Multiple two-terminal device structures have been fabricated and electrical tests were carried out to confirm the extremely high resistivity values of ZnO nanoplates. This is quite unexpected because the chemical synthesis procedure would usually introduce a high density of structural defects in the nanoplates that are usually expected to increase the conductivity of ZnO. Furthermore, ZnO is well known to exhibit n-type semiconducting property with relatively higher electrical conductivities compared to other oxide semiconductors.24 Although not shown here, by employing a photoluminescence spectrometer (PerkinElmer LS-55), photoluminescence (PL) spectra at various excitation wavelengths from 255 to 375 nm have indeed identified the presence of many defect levels including Zn interstitials and oxygen vacancies, and the red-shifted near-band-edge (NBE) emission. This indicated more abundant presence of these structural defects within the nanoplates than in the nanorods. Oxygen molecules are easily adsorbed onto these defects by capturing free electrons below the surface, which can enlarge the electron depletion region.

Upon the irradiation of ultraviolet (UV) light, a tremendous change in the electrical current was observed. The conduction current in the nanoplates under an electrical bias of 10 V increased sharply to 11.7 nA from 0.04 pA, giving a photocurrent to dark current ratio of 2.9 × 105. This ratio is comparable to ∼5 × 105 measured with granular ZnO nanowires by Liu et al.25 and higher than 5 × 104 measured with single crystalline ZnO nanowires by Soci et al.26, which are the highest reported values to the best of our knowledge. For comparison, the current flow through the nanorod was also measured in the present work with and without UV irradiation and plotted in Fig. 2(b). The photocurrent was found to increase by a few times compared to the dark current in a nanorod. An increased current flow by UV irradiation was measured for six different wavelengths of UV ranging from 265 to 395 nm as plotted in Fig. 2(c), in which the photocurrent increase was found to be more effective for UV of short wavelengths. On the other hand, an opposite trend was found for the nanorods as shown in Fig. 2(d).

It is conjectured that the measured electrical current through a nanoplate is affected by the adsorption and desorption of water and oxygen molecules from the atmosphere at the surface of the nanoplates.27 The adsorbed molecules couple with free electrons near the surface immobilizing them to effectively increase the resistivity of the conduction channels close to the surface. When UV irradiation removes the surface adsorbed molecules, electrons are released and contribute to the increased photocurrent. This mechanism of electron conduction near the surface should apply for both nanoplate and nanorod devices. In the case of nanoplates, however, their morphology provides a much higher efficiency for the formation of near-surface resistive channels of the charge carriers due to the high surface to volume ratio. The thickness of a nanoplate is tens of nanometers, whereas a nanorod has a width of thousands of nanometers. Therefore, the entire volume of the nanoplates can be considered to work as the conduction channels under the influence of surface adsorption and desorption of oxygen molecules, thereby the change of resistivity by UV irradiation should be much more efficient.

In addition to the thickness of nanoplates, the crystal-plane-dependent photo-response of 1D ZnO nanorods and 2D ZnO nanoplates should be considered, although both nanorod and nanoplate shapes were derived from the identical crystal structure. From the Wurtzite structure of ZnO and the atomic arrangement of Zn and O atoms, the top and bottom surfaces of the nanoplates correspond to the polar (0001) planes while the external surfaces of nanorod sidewalls are nonpolar (10[1 with combining macron]0) or (1[1 with combining macron]00) planes.28 Therefore, adsorption and desorption of molecules should occur more actively on the polar (0001) surface of ZnO nanoplates, where a significant amount of oxygen and water molecules in air can adsorb.29,30 In order to check the roles of these environmental factors, we measured the photocurrents under various environmental conditions inside the measurement chamber with a controlled atmosphere and temperature. As the temperature of the nanoplates increased from 300 to 380 K, the corresponding increase of the photocurrent could be observed (Fig. 3(a)). It is noted that measurements in the chamber were performed with a reduced intensity of UV light due to the limitation of the closed chamber. The obtained result indicates that the evaporation of water molecules above 373 K results in the reduced adsorption of water molecules on the (0001) surface, making more photogenerated electrons available. Measurements of the photocurrent under the reduced atmospheric pressure could also confirm the effect of the reduced amount of surface adsorbed molecules. As shown in Fig. 3(b), the photocurrent by UV irradiation was found to be greatly enhanced in a vacuum of a few mTorr pressure. Similar behavior was analysed with the addition of a N2 flow. The photocurrent to dark current ratio exhibited a high value of ∼1.8 × 105 at a lower UV intensity, and this is thought to be due to the scarcity of gas molecules to be adsorbed on the surface in the atmosphere. The vacuum or N2 atmosphere resulted in the detachment of the adsorbed oxygen and water molecules from the nanoplate surfaces, thereby further reducing an electron depletion layer. Decreased adsorption of ambient molecules affects the current flow through the depletion layer even without the UV irradiation, increasing the dark current as well, but the desorption of the molecules with UV illumination works more effectively than in air. As a result, a further improvement with higher photocurrent to dark current ratios is possible with controlled environments that reduce the influence of gas molecules on the surface of nanoplates from the atmosphere.


image file: c8nr00569a-f3.tif
Fig. 3 Current increase with UV irradiation at 0.15 mW cm−2 was measured at different temperatures (a), and under different atmospheric conditions (b). The results are summarized in (c), where the dark current (black dot) and photocurrent (red dot) under each measuring condition (from A to F, whose details are in the table) were compared.

Relatively slower responses of current changes with UV light off under these conditions as shown in Fig. 3(b) can also be understood from the reduced amount of surface adsorption in the low pressure or inert N2 atmosphere. When air was introduced to the surface of the nanoplates, it was observed that the nanoplate quickly returns to its high resistance state. As these procedures are mediated via electrostatic interactions, the polar surfaces of ZnO nanoplates are more sensitive to ambient gas molecules, and the photoelectric properties of ZnO nanoplates can be significantly improved by using these responses.31

In summary, we report an economical and facile synthesis method to grow single crystalline nanoplates of ZnO with high width/thickness aspect ratios of greater than 100. The growth mechanism is based on the epitaxial nucleation of ZnO on the surface of GaN where lattice mismatches between crystalline facets seem to determine the preferred growth directions of ZnO nanostructures. It is demonstrated that a thin ZnO nanoplate grown in parallel with the basal plane of the hexagonal crystal structure exhibits a significantly higher resistivity compared to ZnO in bulk or nanorod forms, but extremely high UV light sensitivity of the electrical resistivity. The obtained results were understood based on the surface adsorption and desorption of atmospheric molecules on the polar surfaces of the nanoplates, resulting in the formation of an electron depletion region near the (0001) surfaces. The high density and high sensitivity of these surfaces in the ZnO nanoplates suggest the possible applications of nanoplates as photodetector devices with utmost sensitivities.

Conflicts of interest

There are no conflict of interests to declare.

Acknowledgements

The authors acknowledge the financial support from the National Research Foundation (NRF) of Korea under the grant numbers of 2012K1A4A3053565 and 2017R1A2B4003139, and from the DGIST research program of 17-BT-02.

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