Photocurrent detection of chemically tuned hierarchical ZnO nanostructures grown on seed layers formed by atomic layer deposition
© Bang et al.; licensee Springer. 2012
Received: 5 April 2012
Accepted: 31 May 2012
Published: 6 June 2012
We demonstrate the morphological control method of ZnO nanostructures by atomic layer deposition (ALD) on an Al2O3/ZnO seed layer surface and the application of a hierarchical ZnO nanostructure for a photodetector. Two layers of ZnO and Al2O3 prepared using ALD with different pH values in solution coexisted on the alloy film surface, leading to deactivation of the surface hydroxyl groups. This surface complex decreased the ZnO nucleation on the seed layer surface, and thereby effectively screened the inherent surface polarity of ZnO. As a result, a 2-D zinc hydroxyl compound nanosheet was produced. With increasing ALD cycles of ZnO in the seed layer, the nanostructure morphology changes from 2-D nanosheet to 1-D nanorod due to the recovery of the natural crystallinity and polarity of ZnO. The thin ALD ZnO seed layer conformally covers the complex nanosheet structure to produce a nanorod, then a 3-D, hierarchical ZnO nanostructure was synthesized using a combined hydrothermal and ALD method. During the deposition of the ALD ZnO seed layer, the zinc hydroxyl compound nanosheets underwent a self-annealing process at 150 °C, resulting in structural transformation to pure ZnO 3-D nanosheets without collapse of the intrinsic morphology. The investigation on band electronic properties of ZnO 2-D nanosheet and 3-D hierarchical structure revealed noticeable variations depending on the richness of Zn-OH in each morphology. The improved visible and ultraviolet photocurrent characteristics of a photodetector with the active region using 3-D hierarchical structure against those of 2-D nanosheet structure were achieved.
Nanostructured materials, which are defined as materials with structural elements, such as clusters, crystallites or molecules, with dimensions in the 1 to 100-nm range, have been the interest of both academic and industrial fields over the past few decades because nanosize scaling allows materials to exhibit novel and significantly improved physical, chemical, and biological properties [1–3]. In addition, nanostructures can provide unprecedented understanding on materials and devices. For these reasons, fabrication methods and characterizations of various nanostructured materials have been extensively investigated. Zinc oxide (ZnO), with a wide band gap (3.37 eV), has been actively studied due to its excellent chemical/electrical/optical properties and the ease of nanostructure growth applicable to nanoscale functional devices, such as sensors, solar cells, light emitting diodes, and ultraviolet lasers [4–7]. It is important to fabricate and control the nanostructure size, density, and shape to produce ZnO nanostructures for specific purposes. ZnO nanostructures in the shapes of rods, belts, nails, tubes, stars, and flowers have been prepared by the thermal evaporation of zinc powder and hydrothermal synthesis [8–12]. Metal organic chemical vapor deposition, spray pyrolysis, laser ablation, sputter deposition, and template-assisted growth synthesis methods are typically employed for these nanostructures. In particular, synthesis of ZnO via a chemical solution route provides an easy and convenient method and is very effective for scale-up, even at low temperature. In this hydrothermal method, various shapes and dimensions of ZnO nanostructures can be obtained by tuning the pH, process temperature, concentration of precursors, and seed layer [13–16]. Among them, the seed layer plays an important role in promoting high-density nucleation through reduction of the thermodynamic barrier . Although previous results have demonstrated methods for successful control of the ZnO nanostructure shape, there are additional challenging factors in ZnO nanostructure growth such as (a) preparation of a pure ZnO chemical composition without undesired element incorporation from a seed layer and (b) production of preferential structural growth over large areas [17–19]. The latter factor is crucial to maximize reactive sites of ZnO in specific orientations for surface chemical applications (e.g., gas sensor and heterogeneous catalysis supports). In this regard, achievement of large areas with a (100) ZnO surface orientation is very useful as it has reactive O- and Zn-polar sites [20–22]; however, this part remains a technical challenge in ZnO nanostructure.
In this study, we reported a change in chemical and physical properties of the ZnO nanostructure as shaping ZnO from two-dimensional (2-D) nanosheet to one-dimensional (1-D) nanorod by tuning the Al2O3/ZnO thin bilayer film of the seed layer. The preparation of 2-D ZnO nanostructures is still challenging as ZnO exhibits structural polarity, which induces highly anisotropic c-axis oriented growth. To create various shapes of ZnO nanostructures, thin Al2O3 was applied to screen the inherent polarity of ZnO to serve as an amorphizing layer . The seed layers were deposited by atomic layer deposition (ALD) to precisely control the seed layer. ALD is a thin film growth technique that is based on self-limiting and surface reactions, resulting in films deposited in a layer-by-layer fashion. These features can offer the unique capability to coat complex 3-D nanostructure substrates with a precise and conformal layer, even if prepared at low processing temperatures. We finally introduce a combined ALD and hydrothermal synthesis approach by systematic assembly of 3-D hierarchical nanostructures, constructed using sequential loading of nanorods on nanosheets, revealing finely tuned ZnO-like chemical and physical properties by suppressing hydroxyl groups. These 3-D hierarchical nanostructures are proven to enhance the sensitivity of nanoscale ZnO optical sensor greatly.
The formation of seed layers was performed on a 100-nm-thick SiO2/Si wafer by ALD. First, 5-nm-thick Al2O3 was deposited using trimethylaluminum (Al(CH3)3) and deionized water (H2O) as Al and oxidant precursors, respectively. Then, 1-, 6-, 12-, and 18-nm-thick ZnO films were deposited on 5-nm-thick ALD Al2O3 film using diethylzinc (Zn(CH2CH3)2) and deionized water as Zn and oxidant precursors, respectively. Argon was used as a carrier and purge gas. The process temperature and pressure were 150 °C and 0.5 Torr, respectively. The growth rates were 0.1 for Al2O3 and 0.2 nm/cycle for ZnO films. After coating SiO2 substrates with ZnO on Al2O3 by ALD at various thicknesses, hydrothermal growth of ZnO was performed by suspending the sample upside-down in a Teflon beaker filled with an equimolar aqueous solution (0.02 M) of zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 99.0% purity; Sigma Aldrich, Seoul, South Korea) and hexamethylenetetramine (HMT, C6H12N4, 99.0% purity; Sigma Aldrich). Before introducing the substrate into the growth solution, the Teflon beaker containing the precursor solution was maintained in a laboratory oven at 90 °C for 1 hr to reduce the density of free-floating ZnO nanoparticulates. The substrate was then placed in a heated solution and held at the same temperature for 2 hr. At the end of the growth period, the sample was removed from the solution, then immediately rinsed with deionized water to remove residual salt from the surface. Finally, the sample was dried naturally in laboratory air at room temperature. Therefore, the observed changes in the ZnO nanostructure shape were only related to the changes in the seed layer surface produced by control of the ALD ZnO thickness. The morphological characterizations were obtained using a field emission scanning electron microscope (S-4800, Hitachi, Seoul, South Korea) and transmission electron microscopy (TEM, JEM-3010TEM; JEOL Ltd., Akishima, Tokyo, Japan). The crystal structures were determined by X-ray diffraction (XRD, DMAX-2500; Rigaku, Corporation, Tokyo, Japan) with Cu Kα radiation, and the changes in the chemical bonds of the ZnO nanostructures were analyzed using X-ray photoelectron spectroscopy (XPS, ESCA Lab-2220I; VG Semicon, East Grinstead, Weat Sussex, UK) with a Mg source. The binding energy of each element was calibrated using C-C bonds (284.5 eV) in the C 1 s binding state. The optical property is characterized using a UV–VIS spectrophotometer (U-3010, Hitachi). In order to investigate the photo responsibility, 100-nm-thick SiO2/Si wafers were used as the substrate for photodetector fabrication. Al2O3/ZnO (50/10 nm) channel layers were deposited by ALD. Channel layers were patterned by the lift-off method. Metal electrodes comprised of Ti/Au (30/100 nm) were deposited by an e-beam evaporator. The width and length of the channel layer were 400 and 100 μm, respectively. Next, nanostructures were synthesized on the channel layer. The electrical characteristics of individual nanostructure photodetectors were measured by an Agilent B1500A semiconductor analyzer (Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
The fabrication of (a) thin and relatively pure 2-D nanosheets without collapse of its intrinsic morphology and (b) 3-D hierarchical ZnO nanostructures compared to nanosheets has the following potential advantages in various fields. The ultrathin ZnO nanosheets can lead to enhanced gas and/or organic molecule adsorption due to the larger specific surface area. Additionally, the thin, sheet-like structures can enhance the transportability of light-induced charges from the surface to the inside due to the limited thickness (<20 nm). With regard to the mechanical properties, the aggregation of porous, net-like arrangements of ZnO nanosheets can be effectively prevented by coating with the dense ALD ZnO films to maintain the original, large active surface area. As an interesting potential application of this finding, the large surface area and interspaces of the 3-D hierarchical ZnO nanostructures may offer improved diffusion and mass transportation of molecules and charges in photochemical reactions.
The nanostructure morphology change from a 2-D nanosheet to 1-D nanorod was controlled by changing the seed layer surface. During the ALD of the seed layer, ZnO and Al2O3 hydroxyls coexisted on the alloy film surface, leading to deactivation of the surface hydroxyl groups. This surface complex decreases pure ZnO nucleation on the seed layer surface. Additionally, a thin amorphous Al2O3 layer disrupts the crystalline continuity of the subsequent ALD ZnO, thereby effectively screening the inherent surface polarity of the ZnO and finally inducing a 2-D zinc hydroxyl compound nanosheet formation on the seed layer. With increasing ALD cycles of ZnO in the seed layer, the morphology of the nanostructure changes from a 2-D nanosheet to 1-D nanorods by recovering the natural crystallinity and polarity of ZnO. Thus, the nanorod morphology greatly depends on the property of the substrate surface or seed layer surface, which is considered a key factor. The thin ALD ZnO nucleation layer conformally covered the knotty nanosheet to produce the nanorod. During the ZnO seed layer formation, a zinc hydroxyl compound nanosheet undergoes self-annealing at 150 °C, resulting in morphological transformation to a pure ZnO nanosheet without the collapse of its intrinsic morphology. This 3-D hierarchical nanostructure revealed finely tuned ZnO-like chemical and physical properties by eliminating hydroxyl groups preexisting 2-D nanosheet. The 3-D hierarchical nanostructures are also proven to improve the sensitivity of nanoscale ZnO-based optical sensor. Therefore, this study demonstrates that ALD is a unique approach for changing the surface polarity of a seed layer and conformal hydrothermal nucleation layer formation, creating complex mixtures of 2-D nanosheets with 1-D nanorods.
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (grant no. 2011–0015436).
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