Polar ZnO Surfaces Research Articles

Critical island size for Ag thin film growth on ZnO (0 0 0 [formula omitted

Island growth of Ag on ZnO is investigated with the development of a new technique to approximate critical island sizes. Ag is shown to attach in one of three highly symmetric sites on the ZnO surface or initial monolayers of grown Ag. Due to this, a lattice based adaptive kinetic Monte Carlo (LatAKMC) method is used to investigate initial growth phases. As island formation is commonly reported in the literature, the critical island sizes of Ag islands on a perfect polar ZnO surface and a first monolayer of grown Ag on the ZnO surface are considered. A mean rate approach is used to calculate the average time for an Ag ad-atom to drop off an island and this is then compared to deposition rates on the same island. Results suggest that Ag on ZnO (0001¯) will exhibit Stranski–Krastanov (layer plus island) growth.

Tuning the Band Bending and Controlling the Surface Reactivity at Polar and Nonpolar Surfaces of ZnO through Phosphonic Acid Binding.

ZnO is a prime candidate for future use in transparent electronics; however, development of practical materials requires attention to factors including control of its unusual surface band bending and surface reactivity. In this work, we have modified the O-polar (0001̅), Zn-polar (0001), and m-plane (101̅0) surfaces of ZnO with phosphonic acid (PA) derivatives and measured the effect on the surface band bending and surface sensitivity to atmospheric oxygen. Core level and valence band synchrotron X-ray photoemission spectroscopy was used to measure the surface band bending introduced by PA modifiers with substituents of opposite polarity dipole moment: octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (F13OPA). Both PAs act as surface electron donors, increasing the downward band bending and the strength of the two-dimensional surface electron accumulation layer on all of the ZnO surfaces investigated. On the O-polar (0001̅) and m-plane (101̅0) surfaces, the ODPA modifier produced the largest increase in downward band bending relative to the hydroxyl-terminated unmodified surface of 0.55 and 0.35 eV, respectively. On the Zn-polar (0001) face, the F13OPA modifier gave the largest increase (by 0.50 eV) producing a total downward band bending of 1.00 eV, representing ∼30% of the ZnO band gap. Ultraviolet (UV) photoinduced surface wettability and photoconductivity measurements demonstrated that the PA modifiers are effective at decreasing the sensitivity of the surface toward atmospheric oxygen. Modification with PA derivatives produced a large increase in the persistence of UV-induced photoconductivity and a large reduction in UV-induced changes in surface wettability.

Counterintuitive Reconstruction of the Polar O-Terminated ZnO Surface with Zinc Vacancies and Hydrogen.

Understanding the structure of ZnO surface reconstructions and their resultant properties is crucial to the rational design of ZnO-containing devices ranging from optoelectronics to catalysts. Here, we are motivated by recent experimental work that showed a new surface reconstruction containing Zn vacancies ordered in a Zn(3 × 3) pattern in the subsurface of (0001)-O-terminated ZnO. Reconstruction with Zn vacancies on (0001)-O is surprising and counterintuitive because Zn vacancies enhance the surface dipole rather than reduce it. In this work, we show using density functional theory (DFT) that subsurface Zn vacancies can form on (0001)-O when coupled with adsorption of surface H and are in fact stable under a wide range of common conditions. We also show that these vacancies have a significant ordering tendency and that Sb-doping-created subsurface inversion domain boundaries (IDBs) enhance the driving force of Zn vacancy alignment into large domains of the Zn(3 × 3) reconstruction.

Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single-Crystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting

This work investigates the anisotropic PEC performances of ZnO single-crystalline (SC) photoanodes and the effect of internal electrostatic fields on the separation of photogenerated charge carriers during PEC water splitting. It was found that the internal electrostatic field can greatly influence the bulk charge separation efficiencies during PEC water splitting depending on its orientations, which can only be promoted as the internal electrostatic field in accordance with the direction of the holes’ transportation. Due to the surface stabilization of ZnO polar surfaces, the internal electrostatic field would be gradually decreased to zero near the surface of ZnO SC photoanodes. Therefore, the interfacial charge separation would be mainly determined by the interfacial electric fields in the space charge region formed by the equilibration of the Fermi levels between ZnO and the electrolyte solution. However, the differences on the bulk charge separation efficiencies of ZnO SC photoanodes are much larger ...

Surface-assisted defect engineering of point defects in ZnO

Semiconductor surfaces facilitate the injection of highly mobile point defects into the underlying bulk, thereby offering a special means to manipulate bulk defect concentrations. The present work combines diffusion experiments and first-principles calculations for polar ZnO (0001) surface to demonstrate such manipulation. The rate behavior of oxygen interstitial injection varies dramatically between the Zn- and O-terminated ZnO surfaces. A specific injection pathway for the Zn-terminated surface is identified, and activation barrier determined from the first-principles calculations agrees closely with the experimental activation energy of 1.7 eV.

New approaches for calculating absolute surface energies of wurtzite (0001)/(0001¯): A study of ZnO and GaN

The accurate absolute surface energies of (0001)/(0001¯) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating the dangling bond energy of asymmetrically bonded surface atoms. In this study, we used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches. The surface energies of (0001)/(0001¯) surfaces of wurtzite ZnO and GaN that we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group-I and group-VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces, and these results were also compared with the above method. The calculated results generally show that the surface energies of GaN are higher than those of ZnO, suggesting that ZnO tends to wet the GaN substrate, while GaN is unlikely to wet ZnO. Therefore, it will be challenging to grow high quality GaN thin films on ZnO substrates; however, high quality ZnO thin film on GaN substrate would be possible. These calculations and comparisons may provide important insights into crystal growth of the above materials, thereby leading to significant performance enhancements in semiconductor devices.

Mechanism and energetics of O and O2 adsorption on polar and non-polar ZnO surfaces.

Polar surfaces of semiconducting metal oxides can exhibit structures and chemical reactivities that are distinct from their non-polar surfaces. Using first-principles calculations, we examine O adatom and O2 molecule adsorption on 8 different known ZnO reconstructions including Zn-terminated (Zn-ZnO) and O-terminated (O-ZnO) polar surfaces, and non-polar surfaces. We find that adsorption tendencies are largely governed by the thermodynamic environment, but exhibit variations due to the different surface chemistries of various reconstructions. The Zn-ZnO surface reconstructions which appear under O-rich and H-poor environments are found to be most amenable to O and O2 adsorption. We attribute this to the fact that on Zn-ZnO, the O-rich environments that promote O adsorption also simultaneously favor reconstructions that involve adsorbed O species. On these Zn-ZnO surfaces, O2 dissociatively adsorbs to form O adatoms. By contrast, on O-ZnO surfaces, the O-rich conditions required for O or O2 adsorption tend to promote reconstructions involving adsorbed H species, making further O species adsorption more difficult. These insights about O2 adsorption on ZnO surfaces suggest possible design rules to understand the adsorption properties of semiconductor polar surfaces.

Formation and Stability of Phenylphosphonic Acid Monolayers on ZnO: Comparison of In Situ and Ex Situ SAM Preparation.

Self-assembled monolayers (SAMs) enable an electronic interface tailoring of conductive metal oxides and offer an alternative to common transparent electrodes in optoelectronic devices. Here, the influence of surface orientation and pretreatment on the formation and stability of SAMs has been studied for the case of phenylphosphonic acid (PPA) on ZnO single crystals. Using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) and density-functional theory (DFT) calculations, the thermal stability and orientational ordering of PPA-SAMs on the polar and mixed-terminated ZnO surfaces were analyzed. On all surfaces, PPA-SAMs remain stable up to 550 K, while at higher temperatures a C-P bond cleavage and dissociative desorption takes place yielding two distinct desorption peaks. Based on DFT calculations, these desorption channels are attributed to protonated and deprotonated chemisorbed PPA molecules, which can be related to tri- and bidentate species, hence allowing to determine their relative abundance from the intensity ratio. Beside immersion, an alternative monolayer preparation based on vacuum deposition in combination with controlled desorption of excess multilayers is demonstrated. This enables a SAM preparation on bare ZnO surfaces without any precoating due to exposure to ambient air, which is further compared with SAM formation on intentionally hydroxylated substrates. Corresponding TDS data indicate that initial hydroxylation favors the formation of tridentate and deprotonated bidentate, while the OMBD preparation on bare surfaces yields a larger fraction of protonated bidentate species. The orientation of PPA molecules adopted in the SAMs was determined from the dichroism of K-edge NEXAFS measurements and reveals an almost upright orientation for the deprotonated species, while a slight tilting is obtained for monolayer films with a large fraction of protonated bidentate molecules.

Observation of Two Dimensional Metallic Surface States and Metal to Semiconductor Transition on Low Dimensional Polar Thin Films

Two dimensional (2D) metallic conductivity has been studied for more than a decade because the unusual surface metallic states of ultrathin films may result in applications for 2D superconductors, spintronics and optoelectronic devices. Surface metallic states were predicted on ZnO polar surfaces, which is caused by charge transfer from O-terminated (000-1) surface to Zn-terminated (0001) surface to stabilize the polar instability.1 This charge transfer in polar thin films was also found to cause abrupt phase transition (i.e. ferromagnetism,2 and semiconductor to semimetal transition3), opening a new way to design functional devices by engineering surface or interface of ultrathin films. 2D metallic conductivity on ZnO has not been previously demonstrated in experiments. The charge transport properties of 2D surfaces have proved to be difficult to measure because of the dominant effect of bottom, bulk conduction channels. In our study, we reduce the ZnO thickness to several nanometer to suppress the bulk conduction channels and first experimentally show 2D metallic states on polar ZnO surfaces through surface conductivity measurements on SiO2 substrates. Then, introduction of a high density of oxygen vacancies to the polar ZnO surfaces can cause metal to semiconductor transition. These experimental results are also confirmed by the first principle calculations. [1] Wander, A.; Schedin, F.; Steadman, P.; et al., Phys. Rev. Lett. 2001, 86, (17), 3811-3814. [2] Wang, X. R.; Li, C. J.; Lu, W. M.; et al., Science 2015, 349, (6249), 716-719. [3] Kim, J.; Baik, S. S.; Ryu, S. H.; et al., Science 2015, 349, (6249), 723-726.

In situ transmission electron microscopy observation of ZnO polar and non-polar surfaces structure evolution under electron beam irradiation

Using in situ transmission electron microscopy, we investigated the dynamic reconstruction and evolution of ZnO polar and non-polar surfaces under high-energy electron beam irradiation. Electron beam radiolysis creates oxygen vacancies and a Zn rich (0001) surface. Positive polar charges at the (0001) surface expel loosely bonded Zn ions to diffuse away from the (0001) polar surface. As a result, mass loss was observed around the (0001) surface. Dehydration by the electron beam breaks the charge balance on the (0001¯) polar surface. The negative charges on the (0001¯) surface suppress the radiolysis effect and further absorb Zn ions to the surface to neutral the polar charges. The ideal stacking sequences of Zn ions in hexagonal ZnO structure can be considered as ABAB… along its c axis, while the absorbed individual Zn ion on the (0001¯) surface occupies the C site to form three bonds with surface O ions beneath, instead of one bond in the ideal structure. With more Zn ion absorption and surface oxidization, new nanocrystals grow up from the (0001¯) polar surface. New nanocrystals nucleated at the (011¯0) non-polar surface are driven by the electric field of the polar charges as well, for the Zn ions were always observed to absorb on the negatively charged [0001¯] end of the newly formed (011¯0) surface layer.

Ab initio study of the p-hole magnetism at polar surfaces of ZnO: the role of correlations

A standard local density approximation and its self-interaction corrected version are applied to study spontaneous magnetization, promoted by localized p electron holes, of polar oxygen-terminated ZnO surfaces. The electronic properties and magnetic exchange interactions of three different facets are calculated. It is demonstrated that partially filled oxygen p orbitals of the polar surfaces exhibit magnetic moment formation and long range magnetic order leading to the occurrence of a ferromagnetic ground state. Monte Carlo simulations predict Curie temperatures above room temperature. In contrast to isolated defects in bulk materials, applying correlation corrections to the localized p-like surface states does not lead to a collapse of magnetic interaction: as the weakening of the magnetic interaction, caused by the reduced electronic overlap, is compensated by a strengthening due to an increase of the magnetic moments, the ferromagnetism can principally persist above room temperature, provided a large hole concentration exists.

Growth, structure, and electronic properties of nonpolar thin films on a polar substrate:Cr2O3on ZnO (0001) and ZnO (0001¯)

The growth and geometric and electronic structures of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ layers on the polar ZnO surfaces were characterized to determine how polar substrates can influence the properties of nonpolar films. X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), high resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction, low energy electron diffraction, x-ray diffraction (XRD), and x-ray reflectivity (XRR) were employed to characterize the growth mode, film quality, and interfacial electronic properties. Chromium oxide growth on ZnO $(000\overline{1})$ and $(0001)$ followed the same trends: two-dimensional growth with initial disorder followed by the formation of epitaxial ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3} (0001)$. Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ lattice extending all the way to the interface. This indicates that above a critical thickness of 10--15 $\mathrm{Cr}\ensuremath{-}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{Cr}$ repeat units, the entire film reorganizes into an ordered structure. It is postulated that the oxygen remained ordered throughout the growth but that the chromium initially filled interstices randomly in the oxygen sublattice, which allowed the film to eventually grow with a well-defined epitaxial relationship with the substrate. The polar interfaces showed a small band offset that decayed with increasing film thickness, suggesting that the compensating charges at the interface may partially migrate to the film surface. No evidence of formal changes in the Cr oxidation state at the interfaces was seen. On the other hand, statistical analyses of UPS valence band spectra revealed an enhanced density of states near the valence band edge for ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ on ZnO $(0001)$, consistent with stabilization of the positive interface by filling surface electronic states. In contrast, no significant valence band differences were observed between bulk ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and thin ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ layers on ZnO $(000\overline{1})$, suggesting a different charge compensation mechanism on the negative surface. The potential impact of these findings on the surface properties of chromium oxide thin films is discussed.

Hydrogen sensing properties of the [formula omitted] surface enhanced by Be doping: A first principles study

The effect of hydrogen adsorption on the electronic structure and properties of clean and Be-doped ZnO(0 0 0 1¯) polar surface was investigated using the first principles method. We found that the Be-doped surface is more stable than the undoped one, and that high Be solubility can be achieved in ZnO under O-rich condition with the use of metallic Be as a Be doping source. On the other hand, the obtained results show that hydrogen adsorption is more favored on Be-doped− ZnO(0 0 0 1¯) surface than onto the clean surface, and this make Be-doped ZnO and BexZn1−xO more efficient for H gas sensing applications than clean ZnO surface. Our results show also that the Be-doped surface is more efficient for hydrogen storage applications than the clean surface. We have also examined the effect of H adsorption on the surface work function of both clean and Be-doped surfaces. We found that the adsorption of hydrogen molecule increases the work function of both surfaces, while, the adsorption of atomic hydrogen decreases significantly the surface work function.

Piezoelectricity and electronic structures of ZnO thin films: A density functional theory study

The piezoelectric polarization of ZnO (0001) thin films is calculated for varying thickness. A new depolarization mechanism based on the occupation of quantum states induced by the internal electric field is proposed to explain the asymptotical deviation of piezoelectric polarization of thin films from the bulk value. The mechanism is also related to the stability of ZnO polar surfaces. The internal electric field is also demonstrated to have important influence on the defect electronic structures of vacancy-doped ZnO thin films.

The role of ammonia hydroxide in the formation of ZnO hexagonal nanodisks using sol–gel technique and their photocatalytic study

ZnO nanomaterials with large surface area are desired particularly for the gas sensor, biosensor and photocatalyst applications. In this study, ZnO hexagonal nanodisks with thickness to diagonal aspect ratio (∼1/80) were successfully synthesised via sol–gel approach. By using aluminium sulphate as a complexing agent and carefully controlling the amount of ammonia hydroxide, zinc oxide hexagonal nanodisks were produced. The ZnO nanodisks had perfect hexagonal shape with about 4 μm in diagonal and 50 nm in thickness. The growth of the nanodisks was favoured along the six symmetric directions of ±[1ī00], ±[01ī0] and ±[10ī0]. The growth mechanism of ZnO hexagonal nanodisks is proposed as follows. The formation of ZnO hexagonal nanodisks was mediated by the adsorption of aluminate ions, Al(OH)4−, on the polar surface of ZnO. The Al(OH)4− ions were produced as a result of reaction between Al2(SO4)3 and NH4OH. The Al(OH)4− ions were bonded to the positively charged Zn2+-terminated (0001) polar surface of ZnO. This suppressed the preferential growth of ZnO along [0001] direction but allowed the lateral growth of ZnO in <01ī0>. Eventually, ZnO hexagonal nanodisks with ±(0001) top/bottom surfaces and {1ī00} side surfaces were formed. The size of the ZnO hexagonal nanodisks could be adjusted via the synthesis duration and the amount of ammonia hydroxide. The photocatalytic study indicates that ZnO hexagonal nanodisks were a good photocatalyst for the degradation of Rhodamine B under ultraviolet light irradiation with a rate constant of 0.036 min−1.

Structural Evolution of Cu/ZnO Active Sites: From Reactive Environment to Ultrahigh Vacuum

By using first-principles thermodynamics calculations, we investigate the structural evolution of active sites on Cu/ZnO surfaces from reactive environment to ultrahigh vacuum conditions. Under O-rich conditions, the formation of active oxygen vacancies on various ZnO surfaces is unfavorable. However, addition of Cu dopants can significantly improve the reducibility of the ZnO nonpolar and polar surfaces, to an extent that ZnO(0 0 0 1)–O polar surface can be fully reduced. The formed oxygen vacancies in turn enhance the charging of Cu active sites on the highly dispersed metallic monolayers containing Cu and Zn. This is believed to be a factor contributing to the synergetic effects of Cu/ZnO catalysts. Irreversible reconstruction of the active metallic monolayers would take place upon removal from the reactive environment and exposure to ultrahigh vacuum condition, resulting in less active Cu overlayers. Therefore, experiments performed directly under UHV conditions may underestimate the activity of the actual catalysts under reactive environment.

Stabilizing forces acting on ZnO polar surfaces: STM, LEED, and DFT

New understanding has been reached on competing forces acting to stabilize the polar surfaces of intrinsic ZnO. To compensate an accumulating dipole moment normal to ionic planes, the Madelung electrostatic force and the bonding ability of undercoordinated Zn ions compete to deplete more Zn atoms from the (0001) face and more O atoms from the $(\mathbf{000}\overline{\mathbf{1}})$ face. In this competition, the former mechanism wins because it provides very low energy binding sites for O ions at face-centered-cubic registries on both surfaces. On the Zn-face, a distorted tetrahedral structure is formed, while on the O-face, a vertical Y structure is formed. In both structures, O ions form the topmost atomic plane. The reconstructed polar surfaces containing these novel structures have cleavage energy of 2.36 J/${\mathrm{m}}^{2}$, comparable to that of nonpolar surfaces and in agreement with experimental observation. An earlier structure found on the Zn-face annealed at below 1000 K is stabilized mainly by the Madelung electrostatic force and is a metastable structure of that surface.

Large-scale SCC-DFTB calculations of reconstructed polar ZnO surfaces

We present a theoretical study of a range of surface defects for the most abundant polar ZnO(0001)/(0001¯) surfaces using a tight binding approach with self-consistent charges (SCC-DFTB). We find that a combination of triangular pits at the Zn-terminated surface and a strongly ordered hexagonal defect pattern at the O-terminated surface constitutes a very stable reconstruction, in excellent agreement with experimental findings. On the whole, the SCC-DFTB method describes the polar surfaces of ZnO very well, and at a low computational cost which allows for the investigation of larger – and more realistic – surface structures compared to previous studies. Such large-scale calculations show that, at the Zn-terminated surface, the reconstruction results in a high density of one-layer deep triangular pit-like defects and surface vacancies which allow for a high configurational freedom and a vast variety of defect motifs. We also present extensive tests of the performance of the SCC-DFTB method in comparison with DFT results.

The influence of ZnO-binding 12-mer peptides on bio-inspired ZnO formation

ZnO precipitation experiments were carried out in a Trizma (tris(hydroxymethyl)aminomethane)-buffered aqueous solution at 37 °C and in the presence of three ZnO-binding 12-mer peptide pairs, which have nearly the same isoelectric point (pI). With this new approach, the influence of peptide sequences on ZnO mineralization under moderate conditions was investigated. Previous work was focused on electrostatic interactions between inorganic-binding peptides and inorganic surfaces. The precipitates were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the peptide binding to crystallographic planes of ZnO crystals was modeled by molecular dynamics (MD) simulation in explicit water. The binding free energies of all ZnO-binding peptides were calculated. The combined experimental and modeled results demonstrated a direct correlation between the binding strength of the peptide and the morphology of the ZnO particle, due to differences in their specific binding strength towards polar ZnO (001) and nonpolar (100) surfaces. As a consequence, the ZnO-binding peptides inhibited the growth of ZnO crystals by selective adsorption on the polar or nonpolar ZnO surfaces. This was demonstrated by comparison with a precipitation experiment under peptide-free conditions.

Stabilizing reconstruction induced by O protrusions of the ZnO (0001) polar surface

We demonstrated O extrusion to be a universal reconstruction behavior on the Zn-terminated (0001) surface (with or without terraces) to compensate the well-known instability of such a polar surface.