Article pubs.acs.org/Langmuir
ZnO Nanodisk Based UV Detectors with Printed Electrodes Mohammad R. Alenezi,*,§,† Abdullah S. Alshammari,§,‡ Talal H. Alzanki,† Peter Jarowski,§ Simon John Henley,§ and S. Ravi P. Silva*,§ §
Nanoelectronics Center, Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, U.K. College of Technological Studies, PAAET, P.O. Box 42325, Shuwaikh, Kuwait ‡ Department of Physics, College of Science, University of Hail, P.O. Box 2440, Hail, KSA †
ABSTRACT: The fabrication of highly functional materials for practical devices requires a deep understanding of the association between morphological and structural properties and applications. A controlled hydrothermal method to produce single crystal ZnO hexagonal nanodisks, nanorings, and nanoroses using a mixed solution of zinc sulfate (ZnSO4) and hexamethylenetetramine (HMTA) without the need of catalysts, substrates, or templates at low temperature (75 °C) is introduced. Metal−semiconductor− metal (MSM) ultraviolet (UV) detectors were fabricated based on individual and multiple single-crystal zinc oxide (ZnO) hexagonal nanodisks. High quality single crystal individual nanodisk devices were fabricated with inkjet-printed silver electrodes. The detectors fabricated show record photoresponsivity (3300 A/W) and external quantum efficiency (1.2 × 104), which we attribute to the absence of grain boundaries in the single crystal ZnO nanodisk and the polarity of its exposed surface.
■
including microemulsion16 in which sodium bis(2-ethylhexyl)sulfosuccinate was used to decrease the growth along the c-axis. Subsequently, other growth methods were used to produce similar hexagonal disks.17,18 Benefiting from large surface-to-volume ratio and high crystal structure, one- (1D) and two-dimensional (2D) nanostructures are considered ideal platforms to fabricate UV detectors with superior sensitivity, responsivity, and speed of response. Until now, most of the reports on nanostructured UV detectors are based on 1D nanostructures. For example, Hu et al. achieved excellent performance with ultrahigh external quantum efficiency (1.3 × 107) from a thin SnO2 nanowire UV detector, which was attributed to the large surface-to-volume ratio and superior quality of the nanowire.19 Despite the vast literature on the growth and properties of two-dimensional ZnO nanostructures, there are only a few reports on their use in UV detection applications. All reported UV detectors were based on either networks or films of ZnO rather than single ZnO nanodisk as reported in this paper. In this work, the growth of single crystal ZnO hexagonal nanodisks, and the analysis of UV detectors based on single and multiple nanodisks is reported. A facile one-step hydrothermal method to produce single crystal ZnO hexagonal nanodisks, nanorings, and nanoroses without the need of catalysts, substrates, or templates at low temperature (75 °C) is introduced. The morphological and structural characterization
INTRODUCTION Controlled synthesis of nanostructures has progressed greatly in the past few years and had a significant impact on device fabrication and development.1−5 Zinc oxide (ZnO) is one of the dominant materials in nanotechnology, and has been used for a variety of devices such as laser diodes, light-emitting diodes, piezoelectric transducers, and ultraviolet (UV) detectors.6−9 Among these devices, UV photodetectors have been used far and wide commercially and in a number of military applications. These include pollution monitoring, secure space communications, water purification, flame and missile plume detection, etc. High sensor performance (i.e., high sensitivity and short response time) is a key requirement for these uses. Recently, there have been many reports on the growth of ZnO nanostructures using different techniques and their use in UV detection. Solution phase growth methods are attractive for many reasons including simplicity, cost-effectiveness, suitability to large area scalability,1,2 low processing temperature, nonhazardous nature, reproducibility, and compatibility with flexible organic substrates. Hydrothermally grown ZnO nanostructures tend to form 1-D structures due to the faster crystal growth rate along [0001] compared to other directions.10,11 However, by selective adsorption of additives on the planes of the polar crystals, the crystal growth habit can be modified in situ to control the morphology. Different unique shapes have been fabricated via the introduction of different additives, such as citrate ions,12 CTAB,13 and polymers,14,15 into the hydrothermal systems. In this regard, ZnO hexagonal disks and rings were produced via a solution-phase process © 2014 American Chemical Society
Received: January 13, 2014 Revised: March 7, 2014 Published: March 10, 2014 3913
dx.doi.org/10.1021/la500143w | Langmuir 2014, 30, 3913−3921
Langmuir
Article
Figure 1. SEM image of ZnO nanodisk with a diameter of (a) 5 μm, (b) 200 μm, and (c) a stack of ultrathin nanodisks. AFM analysis of (d) a 60 nm thick nanodisk and (e) a stack of ultrathin nanodisks. was used to obtain the conductive silver electrodes. It was noted that the surface of the disk was very hydrophilic, which made it difficult to print on its surface as the ink wets the entire structure. To overcome this problem, the substrate temperature was maintained at 60 °C in order to increase the evaporation rate of the solvent and therefore produce uniform and well-defined electrodes. Subsequently, the device was annealed at 250 °C for 20 min in order to sinter the nanoparticles and to obtain a better contact between the ZnO nanodisk and the silver electrodes. To fabricate the multiple-nanodisks detector, a solution containing low concentration of the nanodisks was drop-cast on a tilted glass substrate, and a coating of multiple nanodisks was formed. Two silver electrodes were made on top of the multiple-nanodiks thin film. For comparison purposes, ZnO thin film fabrication was also performed. First, a 5 mM solution of zinc acetate in ethanol was spin-cast on a glass substrate several times, forming a uniform film of ZnO nanoparticles. The nanoparticle film was annealed at 350 °C for 30 min in air. After it was cooled down, two silver electrodes were deposited on top of the thin film. The electrical characteristics of the fabricated device were recorded using a probe station attached to a Keithley 4200 semiconductor analyzer. The excitation source for the UV detection properties was a monochromatic UV lamp (UVGL-55 hand lamp from UVP LLC) with 50 μW/cm2 intensity at a wavelength of 365 nm.
of these materials is then presented together with a growth mechanism. Finally, single-nanodisk and multiple-nanodisks UV detectors are fabricated, and their performance as fully solution processable large area UV detectors in terms of photosensitivity, photoresponsivity, external quantum efficiency, and response and recovery times is presented.
■
EXPERIMENTAL DETAILS
All reagents in this work were analytical grade. In the typical growth process for ZnO hexagonal nanodisks, a mixture of (100 mM) zinc sulfate (ZnSO4) and (100 mM) HMTA is stirred at room temperature to make a homogeneous solution. The mixture is transferred to a vial and heated to 75 °C in an oven for 3 h. After the vial is cooled down, the solution containing the hexagonal nanodisks is spin-cast onto a clean substrate. The substrate is then dried on a hot plate at 100 °C for 2 h for further characterization. The morphology and crystal structure of the grown structures were observed using a Philips XL-20 scanning electron microscope (SEM) at 10 kV. The grown structures were also examined through powder Xray diffraction (XRD) using a Panalytical X-pert diffractometer with Cu Kα radiation. Scanning transmission electron microscopy (STEM) and electron diffraction characterizations were applied using a Hitachi HD2300A microscope operating at 200 kV. STEM samples were prepared by depositing a drop of diluted suspension of the nanostructure in ethanol on a carbon film coated copper grid. To fabricate the single-nanodisk UV detectors, the solution containing the nanodisks was drop cast onto a clean substrate and silver electrodes printed by an inkjet printer (Dimatix-2831) on top of two edges of the nanodisk. A water-based nanosilver ink (Novacentrix)
■
RESULTS AND DISCUSSION ZnO Nanodisks Growth and Transformation. Figures 1a,b show SEM images of well-defined shaped hexagonal nanodisks grown at 75 °C for 3 h using a 1:1 ratio of ZnSO4 to 3914
dx.doi.org/10.1021/la500143w | Langmuir 2014, 30, 3913−3921
Langmuir
Article
Figure 2. (a) XRD pattern, (b) STEM image, and (c) the corresponding SAED pattern of a single ZnO nanodisk.
HMTA. In each reaction vial we find hexagonal nanodisks with different dimensions, which we classified into three categories by their diameters: 200 μm. At the present time it is not clear why the produced nanodisks have such a large variation in diameters. However, this diameter variation might be a result of different growth periods for catalyzing the different structures. Even though nanodisks grown in the same solution are subject to the same growth conditions, their growth time could vary depending on when their nucleation occurs. In other words, nanodisks that nucleate earlier would have longer growth periods and consequently larger diameter than those which nucleate later. At times the nanodisks are stacked on top of each other as evidenced from SEM images as shown in Figure 1c. AFM analysis shows that the thickness of each hexagonal nanodisk is uniform and ranges from 10 to 100 nm. Figure 1d shows an AFM image of a ND 60 nm thick, where we see the uniformity clearly. A stack of much thinner nanodisks such as the one shown in Figure 1c was also analyzed with AFM (Figure 1e). The stacked nanodisks are 13, 16, 12, and 13 nm in thickness. The XRD pattern of the nanodisks is presented in Figure 2a and clearly indicates that the grown structures are highly crystalline. The diffraction peaks shown are recognized as hexagonal wurtzite-type ZnO. Peaks resulting from impurities were not detected, confirming that the only crystalline material to which the precursor was converted during the reaction was ZnO. The crystallinity of the grown ZnO hexagonal nanodisks was further validated by STEM imaging and electron diffraction. Figure 2b is a typical STEM image of a hexagonal ZnO nanodisk. Even though the diameter is more than 100 μm, the transparency of the ZnO hexagonal nanodisk is evident, indicating its low thickness. Figure 2c depicts the electron diffraction pattern taken when the electron beam was perpendicular to the nanodisk top facet, recognized as the [0001] axis of the hexagonal ZnO lattice. This diffraction analysis suggests that the ZnO nanodisks are single crystal and the dominant exposed facets are the polar {0001} planes. In the hydrothermal process, ZnO has a habit of forming 1-D structures due to the faster growth rate along [0001] compared to those in other directions. Nevertheless, varying the zinc counterions in the growth solution often results in growing crystals with different morphological properties.20 These changes in morphology may arise from the impact of the promoter species blocking nucleation on certain facets. The term ‘“crystal growth inhibition”’ simply refers to the change in the morphology of crystal structure by obstructing growth on one or more facets.21 In the current case, the hexagonal nanodisk morphology is attributable to the anisotropic growth,
where the rate of growing laterally is significantly faster than that in the [0001] direction. The outermost layer of the (0001) facet possesses a positive effective charge, consisting of Zn2+ ions. Therefore, since there are no additives in our hydrothermal reaction, the counterions (SO42¯) in the solution could be adsorbed on the (0001) surface rather than the side nonpolar facets, substituting for hydroxyl anions and hindering the supply of [Zn(OH)4]2− units to the (0001) surface. As a result, the growth of ZnO along the c-axis direction is significantly blocked, forcing the crystal to grow sideways only forming the hexagonal disks (Figures 1a,b). For the sake of comparing the effects of different zinc counterions on the morphological structure, another zinc counterion, Zn(NO3)2, was used. The results, as expected, were one-dimensional ZnO nanostructures shown in Figure 1c. Sulfate and nitrate as zinc counterions show different abilities to cap the polar facet of the growing ZnO nanostructures and as a result lead to different morphologies. It was found that sulfate binds zinc much more energetically than two equivalent nitrates due to the huge difference between their dissociation constants. Using the ChemEQL software, estimates of the dissociation constants in water at a pH of 8 gives 5 orders of magnitude difference between the two counterions (sulfate, Kdis = 0.004, versus nitrate, Kdis = 200) (Scheme 1). Considering zinc-terminated polar facets, it is reasonable to assume that sulfate ions will terminate much more strongly than nitrate, resulting in a strong capping effect and a slowing of the growth of the polar facet by sulfate. This effect will lead to the hexagonal disks observed. Additionally, the binding mechanism of the surface would be significantly different for sulfate and nitrate on this surface. While one sulfate ion can provide full charge balancing of the Zn2+ ion on the surface, two nitrate ions are needed to affect a similar outcome. In solution, these ions would bind the Zn2+ ion from opposite sides, but on the surface they would be forced to bind the Zn2+ ions cofacially, leading to significant steric hindrance between nitrate groups. The outcome will significantly reduce adsorption for nitrate compared to sulfate (KNO3,ad < KSO4,ad) and a poor coverage of the surface with nitrate ions. Further work will be conducted to support this theory using ab initio quantum mechanics and testing a range of counterions of zinc. The growth temperature, time, solution concentration, and the ZnSO4 to HMTA concentration ratio in the growth solution play an important role in the morphology of the produced ZnO structures. It was noted that the shape and size of the hexagonal nanodisks varied with changing growth temperature. Lowering the growth temperature below 60 °C gives thick irregular hexagonal nanodisks with small diameter (
