ZnO Substrates Research Articles

Электронные состояния зоны проводимости ультратонких пленок тиофен-фенилен со-олигомера и замещенного бифенила на поверхности послойно выращенного ZnO

The results of studying the electronic states of the conduction band and interface potential barrier during the formation of ultrathin films of thiophene-phenylene co-oligomer CH3-phenylene-thiophene-thiophene-phenylene-CH3 (CH3-PTTP-CH3) on the surface of ZnO and films of biphenyl tetracarboxylic dianhydride (BPDA) on the ZnO surface are presented. A 100 nm thick ZnO layer was prepared by atomic layer deposition (ALD). Organic CH3-PTTP-CH3 films and BPDA films up to 8 nm thick were formed by thermal vacuum deposition. During film deposition, the electronic characteristics of the surface were studied using total current spectroscopy (TCS) in the energy range from 5 eV to 20 eV above EF. In this energy range, the structure of the maxima of the unoccupied electronic states of CH3-PTTP-CH3 and BPDA films was determined. As a result of the CH3-PTTP-CH3 film deposition, a decrease in the work function to 4.0 eV was found, compared with the value of the work function of 4.2 eV measured from the ALD ZnO substrate. This corresponds to the transfer of a negative charge from the СH3-PTTP-CH3 film to the substrate. The charge transfer at the interface between the BPDA film and the ALD ZnO substrate occurs in the opposite direction, since a 4.7 eV increase of the work function was registered during the formation of this interface. The СH3-PTTP-СH3 and BPDA films studied and the layer-by-layer grown ZnO film represent a continuous coating on sufficiently large surface areas of the order of 10 micrometers x 10 micrometers. The roughness of the ZnO surface does not exceed 4 nm, and the surface roughness of CH3-PTTP-CH3 and BPDA films was 10–15 nm.

Strain-induced formation of self-assembled InGaN/GaN superlattices in nominal InGaN films grown by plasma-assisted molecular beam epitaxy

In this work, we report the spontaneous formation of superlattice structures in nominal InGaN films grown by plasma-assisted molecular beam epitaxy. A 700-nm-thick self-assembled ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}/\mathrm{GaN}$ superlattice with excellent structural quality was achieved. Strain was studied as a possible driving force for the formation of self-assembled superlattice (SASL) structure by growth of InGaN on ZnO substrate using similar growth conditions. The SASL structures were optically characterized using photoluminescence spectroscopy. Structural characterization was conducted via transmission electron microscopy and atom probe tomography. High-resolution x-ray diffraction (XRD) and XRD reciprocal space map were utilized to determine the average composition and the degree of relaxation of InGaN films. We propose that the vertical phase separation observed in the SASL structure is caused by high-temperature growth and intensified by strain. This work provides a method for engineering strain and growth of thick InGaN films for a variety of applications including solar cells and photodetectors.

Field-induced semiconductor–metal transition of hybrid ZnO and graphene nanocomposites

In this study, structural and electronic properties of two-dimensional hybrid ZnO/graphene (ZnO/G) and ZnO/graphene/graphene (ZnO/G/G) nanocomposites are investigated based on first-principles calculations. The calculation indicates that interlayers of the two nanocomposites exist typical van der Waals (vdW) weak interactions. The intrinsic electronic properties of ZnO and graphene monolayer are well-conserved, and the deposition on ZnO substrate opens a band gap of graphene at the Dirac point. When electric fields are applied, the ZnO/G nanocomposites exhibits semiconducting to metallic transition. The transition between Schottky and Ohmic contacts can be effectively implemented, accompanied by the conversion of p-type and n-type Schottky. Meanwhile, the projected density of states and work function of ZnO/G are deeply investigated to discuss the effects of orbital and charge transfer. Compared to ZnO/G, ZnO/G/G would overcome the lower Schottky barrier height (SBH) to form Ohmic contacts, indicating a higher sensitivity to electric field modulation. The formation of low-resistance Ohmic contact has important application for high-performance semiconductor devices.

A multi-platform sensor for selective and sensitive H2S monitoring: Three-dimensional macroporous ZnO encapsulated by MOFs with small Pt nanoparticles

The high-selectivity and high-sensitivity determination of trace concentrations of toxic gases is a major challenge when using semiconductor metal oxide (SMO) gas sensors in complicated real-world environments. In this study, by strategically combining a three-dimensional inverse opal (3DIO) macroporous ZnO substrate and a ZIF-8 outer filter membrane, two series of sensors with Pt NPs loaded at different locations are developed. In the optimal 3DIO ZnO@ZIF-8/Pt sensor, the existence of small Pt NPs in ZIF-8 cavities can effectively accelerate the absorption of H2S, capture electrons from the N site of ZIF-8, and donate the electron to the S site of H2S, as indicated by density functional theory simulations, leading to a significantly increased response to H2S. Together with the molecular-sieving effect that ZIF-8 exerts on gas molecules with larger kinetic diameters, the 3DIO ZnO@ZIF-8/Pt sensor exhibits a high response to H2S (118–5.5 ppm), a detection limit of 40 ppb, and importantly, a 59-fold higher selectivity to H2S against typical interference gases. In addition, the 3DIO ZnO@ZIF-8/Pt sensor is developed as a multi-platform sensor to evaluate trace concentrations of H2S in meat quality assessment, halitosis diagnosis, and automobile exhaust assessment.

A 3nm-thick, quasi-single crystalline Cu layer with ultralow optoelectrical losses and exceptional durability

An ultrathin, highly crystalline, two-dimensional Cu layer is necessary for simultaneously achieving ultralow electrical and optical losses in transparent electrodes. However, perfect Cu wetting on heterogeneous oxide substrates has not yet been achieved. Herein, we report the Ge-mediated fabrication of an ultrathin, highly crystalline, complexly continuous Cu layer. The unique surfactant-like segregation of atomic Ge towards the outermost boundaries of the Cu geometries remarkably enhanced Cu wetting on the ZnO substrate. Numerical simulation indicated that the enhanced wetting can be attributed to the simultaneous decreases in the thermodynamic cohesive and formation energies at the surface and interface of the Cu nanostructures owing to Ge segregation. This enabled the fabrication of an ultrathin (∼3 nm), highly crystalline, two-dimensional Cu layer in a ZnO/Cu/ZnO configuration, which exhibited a record-low average optical loss at a near-bulk resistivity (8 × 10−8 Ω m) comparable to those of conventional Ag electrodes; it also showed exceptional durability in water, ozone, and high-temperature (up to 400 °C) environments. These unique features would ensure the development of highly reliable optoelectrical devices employing Cu superstructure-based transparent electrodes as inexpensive alternatives to Ag-based transparent electrodes.

Evaporation and heat transfer characteristics of CO2, CF4 and CO2/CF4 on [formula omitted], [formula omitted] and [formula omitted] surfaces

This paper investigated the evaporation processes of CO2, CF4, and CO2/CF4 on ZnO(101¯0),ZnO(0001) and ZnO(112¯1) surfaces and the heat transfer characteristics during the evaporation. The molecular dynamics simulations are conducted under atmospheric conditions with 145 K, 195 K and 260 K. The results showed that the evaporation of the three kinds of liquids are the highest on ZnO(112¯1) substrate but the lowest on ZnO(0001) substrate. The evaporation response of CO2 to the temperature has a delay effect compared with that of CF4 and CO2/CF4 mixture. Furthermore, the heat flux fluctuates during the initial evaporation stage, and the temperature increase is more obvious to accelerate the heat transfer balance of CF4/ZnO and mixture/ZnO systems. The number of evaporated CF4 particles in the CO2/CF4 mixed system is higher than that in the pure system, indicating that the presence of CO2 will promote the evaporation of CF4. The boiling phenomenon was found for the systems containing CF4 molecules when the temperature is higher than the phase transition temperature, and the bubbles appeared earliest when ZnO(101¯0) was used as the substrate which makes the heat flux jump obviously in the initial evaporation stage. This study provides insight into the evaporation and heat transfer properties of liquid/solid system with ZnO as the substrate.

Low-defect-density ZnO homoepitaxial films grown by low-temperature ALD

We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO substrates at the relatively low temperatures of 120, 150, and 200 °C. The as-grown ZnO films are studied with aberration-corrected transmission electron microscopy and diffraction contrast, photoluminescence (PL), and surface photovoltage (SPV) spectroscopy. We find that the homoepitaxial films have a monocrystalline structure with the density of basal stacking faults comparable to that of the substrate (∼1011 cm−2) and that the stacking faults can induce high lattice strain due to their interaction with the inversion domain boundaries. The narrow excitonic PL linewidth (2 meV at 8 K) and the sharp SPV bandgap transition confirm the high quality of the ZnO films. Despite similarities in the film properties, the growth temperature has an effect on the density and spatial distribution of intrinsic defects. Our results demonstrate a considerable potential of ALD ZnO homoepitaxy for fabricating high-quality ZnO nanostructures and attaining viable p-type ZnO.

Heteroepitaxial registry and band structures at the polar-to-polar STO/ZnO(000[formula omitted]) interfaces

The interface between ZnO and SrTiO3 (STO) provides a paradigm for combining perovskite oxides and wurtzite-structure semiconductors. However, the heteroepitaxial and energy band structures of the polar-to-polar STO/ZnO(0001¯) interfaces has rarely been studied. In this study, it is shown that the STO films prepared on the ZnO(0001¯) substrate by pulsed laser deposition possess [011]STO and [111]STO azimuth orientations, which exhibit three- and two-fold rotation domains respectively, as demonstrated by X-ray diffraction and transmission electron microscopy. The band structures of the STO(011)/ZnO(0001¯) and STO(111)/ZnO(0001¯) interfaces are bending downward as shown in the I-V characteristic spectra and first principle calculations. These results are different from the single orientation ZnO films grown on STO-(011) and -(111) substrate, as reported in previous literatures, showing the heteroepitaxial asymmetry between STO/ZnO and ZnO/STO interfaces. This work presents an approach towards the physical modeling of combinations between perovskite oxides and wurtzite-structure semiconductors.

Cesium-Induced Active Sites for C-C Coupling and Ethanol Synthesis from CO2 Hydrogenation on Cu/ZnO(0001̅) Surfaces.

The efficient conversion of carbon dioxide, a major air pollutant, into ethanol or higher alcohols is a big challenge in heterogeneous catalysis, generating great interest in both basic scientific research and commercial applications. Here, we report the facilitated methanol synthesis and the enabled ethanol synthesis from carbon dioxide hydrogenation on a catalyst generated by codepositing Cs and Cu on a ZnO(0001̅) substrate. A combination of catalytic testing, X-ray photoelectron spectroscopy (XPS) measurements, and calculations based on density functional theory (DFT) and kinetic Monte Carlo (KMC) simulation was used. The results of XPS showed a clear change in the reaction mechanism when going from Cs/Cu(111) to a Cs/Cu/ZnO(0001̅) catalyst. The Cs-promoting effect on C-C coupling is a result of a synergy among Cs, Cu, and ZnO components that leads to the presence of CHx and CHyO species on the surface. According to the DFT-based KMC simulations, the deposition of Cs introduces multifunctional sites with a unique structure at the Cu-Cs-ZnO interface, particularly being able to promote the interaction with CO2 and thus the methanol synthesis predominantly via the formate pathway. More importantly, it tunes the CHO binding strongly enough to facilitate the HCOOH decomposition to CHO via the formate pathway, but weakly enough to allow further hydrogenation to methanol. The fine-tuning of CHO binding also enables a close alignment of a CHO pair to facilitate the C-C coupling and eventually ethanol synthesis. Our study opens new possibilities to allow the highly active and selective conversion of carbon dioxide to higher alcohols on widely used and low-cost Cu-based catalysts.

Surface Plasmon Resonance from Gallium-Doped Zinc Oxide Nanoparticles and Their Electromagnetic Enhancement Contribution to Surface-Enhanced Raman Scattering

In recent years, semiconductor-based surface-enhanced Raman scattering (SERS) substrates have received considerable attention and led to a forefront in the SERS field. However, the lack of electromagnetic (EM) enhancement contributions highly precludes the development of semiconductive-substrate-based SERS. In this study, Ga-doped ZnO nanoparticles (NPs) were fabricated and employed as novel SERS substrates based on the EM enhancement contribution of surface plasmon resonance (SPR). The obtained Ga-doped ZnO NPs exhibited obvious SPR absorptions in the visible and near- and mid-infrared regions. SPR absorption can be readily tuned by changing the doping ratios of Ga3+ ions. The SERS spectra of Ga-doped ZnO/4-mercaptopyridine (MPy) were investigated at different excitation wavelengths of 488, 532, 633, and 785 nm. The spectral enhancement of Ga-doped ZnO substrates depended on the doping ratios, excitation wavelengths, and nearby SPR absorption. Ga-doped ZnO NPs with the highest free charge carrier density and the doping ratio of 5% showed the strongest SERS spectra. For the fixed doping ratio of 5%, the better is the match between excitation wavelengths and SPR absorption, the higher is the SERS spectral enhancement. This study showed the feasibility of EM contributions to SERS by using semiconductive substrates and can contribute to the development of the semiconductor-based EM mechanism.

Nanoscale structural investigation of Zn1–x Mg x O alloy films on polar and nonpolar ZnO substrates with different Mg contents* *Project supported by the National Natural Science Foundation of China (Grant No. 11804050).

Zn1–x Mg x O alloy films are important deep ultraviolet photoelectric materials. In this work, we used plasma-assisted molecular beam epitaxy to prepare Zn1–x Mg x O films with different magnesium contents on polar (0001) and nonpolar () ZnO substrates. The nanoscale structural features of the grown alloy films as well as the interfaces were investigated. It was observed that the cubic phases of the alloy films emerged when the Mg content reached 20% and 37% for the alloy films grown on the (0001) and () ZnO substrates, respectively. High-resolution transmission electron microscopy images revealed cubic phases without visible hexagonal phases for the alloy films with more than 70% magnesium, and the cubic phases exhibited three-fold and two-fold rotations for the alloy films on the polar (0001) and nonpolar () ZnO substrates, respectively. This work aims to provide references for monitoring the Zn1–x Mg x O film structure with respect to different substrate orientations.

Interface and optical properties of Zn1−xMgxO films with Mg content of more than 70% grown on the (12̄10)-ZnO substrates

Fabricating Zn1−xMgxO films with a high Mg content is key to their applications in deep-ultraviolet optoelectronic devices. In this work, we report the preparation of Zn1−xMgxO films on (12̄10)-ZnO substrates by molecular beam epitaxy. The Zn1−xMgxO/(12̄10)-ZnO structure is revealed by x-ray diffraction and high-resolution transmission electron microscopy. Remarkably, no cubic MgO is observed for films with 74.6% Mg content; the film shows mainly the wurtzite structure with some intermediate phases at the interface. Photoluminescence spectra show that the film exhibits good optoelectronic properties with a bandgap of 4.6 eV. This work provides a new avenue for the fabrication of deep-ultraviolet Zn1−xMgxO films.

Nonpolar short-period ZnO/MgO superlattices: Radiative excitons analysis

The basic structural and detailed analysis of the optical properties of homoepitaxially grown ZnO/MgO superlattices (SLs) on a-oriented ZnO substrates, containing 30 periods of layers, was described. We found using Rutherford backscattering spectroscopy that the wurtzite coordination collapses in the SLs with 3 nm MgO thick barriers. This novel information allows taking this value as the limit for the maximum MgO barrier thickness in such structures. The observed luminescence and the performed calculations of excitonic transitions suggest that such a high energy barrier of MgO allows treating the ZnO layers in these SLs as separate, non-interacting quantum wells. Cross-sectional SEM-CL line scans revealed MgO layer thickness-dependent biaxial strains built up in the QWs structures.

Regulating Ag Wettability via Modulating Surface Stoichiometry of ZnO Substrates for Flexible Electronics

AbstractA novel and highly efficient methodology to regulate (enhance or suppress) the Volmer–Weber 3D growth mode of ultra‐thin (<10 nm) Ag layers by modulating the surface stoichiometry of ZnO substrates prior to Ag deposition is presented. Relative to pristine ZnO layers, oxygen‐deficient surface states formed by preferential removal of surface oxygen atoms remarkably improve Ag layer wettability, whereas oxygen‐excessive surface states formed by oxygen atom incorporation strongly facilitate Ag agglomeration. The dissimilar nucleation and coalescence dynamics are elucidated via combined molecular dynamics and force‐bias Monte Carlo simulations. The improved wettability results in significantly lower sheet resistance in the ultra‐thin (6–10 nm) Ag layers, for example, 6.03 Ωsq−1 at 8 nm, than the previously reported values from numerous other approaches in the equal thickness range. When this unique methodology is applied to ZnO/Ag/ZnO transparent electrodes, simultaneous improvement in electrical conductivity and visible transparency is realized, with a resultant Haacke figure of merit value of 0.139 Ω−1 that is >50% higher than the best reported value for an identically structured electrode. We select transparent heating devices as a model system to confirm that the superior optoelectronic properties are highly sustainable under simultaneous and severe electrical, mechanical, and thermal stresses.

Fabrication and characterization of ZnO:Sb/n-ZnO homojunctions

Antimony-doped ZnO layers have been grown by metalorganic vapour-phase epitaxy on sapphire and ZnO substrates at high-temperature (950 °C) and low-pressure conditions (50 torr). Nitrous oxide and diethyl-zinc have been used as oxygen and zinc precursors, respectively. The incorporation of antimony has been obtained from the decomposition of triethylantimony doping molecules added in the gas phase. High Sb concentrations were measured from 1019 to 1021 at/cm−3 using secondary ion mass spectroscopy and depend on the nature and the orientation on the substrate. Low-temperature photoluminescence spectra of Sb-doped layers exhibit donor–acceptor pair transitions at 3.253 eV. Unlike Raman spectra of nitrogen-doped ZnO layers which show several local vibrational modes related to nitrogen incorporation, these modes were found to be absent in the antimony-doped ZnO layers. Transmission electron microscopy suggests that the incorporation of Sb is partly related to dislocations and other structural defects. All together, the characterizations suggest the formation of acceptor dopant–defect complexes, such as SbZn-2VZn. Finally, ZnO:Sb/n-ZnO homojunction diodes have been successfully elaborated on ZnO substrate. The current–voltage characteristics of the device exhibit a rectifying behaviour with a turn-on voltage of 3 V.

A SERS biosensor constructed by calcined ZnO substrate with high-efficiency charge transfer for sensitive detection of Pb2+

Herein, calcined ZnO submicron flowers (ZnO SFs) with excellent surface enhanced Raman scattering (SERS) activity were employed as substrate to fabricate a novel SERS biosensor for sensitive detection of Pb2+, which improved SERS effect of ZnO submicron particles (ZnO SPs). Compared with ZnO SPs, calcined ZnO SFs substrate possessed narrow energy band to enhance the efficiency of charge transfer owing to easily generate exciton transition to achieve eminent Raman enhancement effect. Furthermore, free-legged DNA walker amplification strategy was utilized to convert trace amounts of Pb2+ into vast trigger DNAs for construction an ultrasensitive biosensor, which exhibited a liner relationship ranged from 10 pM to 100 μM with a low detection limit of 3.55 pM. Therefore, this work not only provided a novel approach for detection of heavy metal ions, but also broadened the application of semiconductor SERS substrate.

Insights into effects of O-incorporated Ag nanoparticles as wetting seeds toward improving Ag wetting on oxides

Ag wetting on oxides is essential for implementing exceptional layered Ag superstructures with simultaneous ultralow losses in electrical conductance and optical transmittance. This goal is difficult to achieve by conventional synthetic techniques involving the incorporation of heterogeneous transition metals or metalloids into Ag as wetting inducers as they cause detrimental optoelectrical changes. This study provides novel insights into the use of homogeneous Ag wetting seeds to improve the wetting of Ag on oxides. We investigated the effect of atomic O incorporation on the improvement of Ag wetting by monitoring its participation in and contribution to the Ag clustering and layering processes. Minimal incorporation of dissolved atomic O as a surfactant into the Ag nanoparticles transposed the nanoparticle evolution path from a complete to incomplete coalescence mode at significantly reduced thicknesses, which was attributed to a reduction in the nanoparticle free energy. The size and shape of the O-incorporated Ag nanoparticles were precisely controlled to accelerate the wetting transition of Ag on ZnO substrates by circumventing the transmutation effect of O. These results demonstrate a notable strategy for accomplishing a rapid Ag wetting transition toward a completely continuous layer with ultralow optoelectrical losses.

Second harmonic generation on excitons in ZnO/(Zn,Mg)O quantum wells with built-in electric fields

Optical second harmonic generation (SHG) on excitons is studied in polar ZnO/(Zn,Mg)O quantum wells grown on ZnO and sapphire substrates. We observe SHG signal on exciton resonances in a symmetry-forbidden geometry for light propagation along the $c$ axis of the structure. A symmetry analysis allows us to suggest that the signals are induced by a symmetry reduction with respect to ZnO bulk crystals by the well-barrier interfaces in combination with built-in electric fields due to spontaneous and piezoelectric polarization as well as structural defects.

Ag-grafted on ZnO nanorod arrays using UV-assisted irradiation for enhanced SERS behavior in CV detection

Introduction: Semiconductor-based surface-enhanced Raman scattering (SERS) substrates with high stability and reproducibility have become one of the essential analytical tools in the analysis of chemical and biological at trace levels. Herein, a growth of the hexagonal-wrapped ZnO nanorod arrays decorating with Ag nanoparticles (AgNPs) at different concentrations of Ag was proposed.
 Methods: The crystallinity, morphology, chemical composition, and optical properties of the prepared samples were investigated by X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Raman system, respectively.
 Results: The results revealed that the SERS performance of ZnO NRs incorporating with AgNPs exhibited higher detection of crystal violet (CV) probe molecules at a low concentration of 10􀀀8 M than that of the pristine ZnO NRs. This effect originates from the localized surface plasmonic resonance of AgNPs that could cause a strong electromagnetic field and synergistic effects of Ag, ZnO, and CV molecules in ZnONRs@Ag/CV SERS system.
 Conclusion: These outcomes reveal that AgNPs play a crucial role in enhanced SERS performance for chemical and biological detection of ZnO substrate.

Influence of Cu(NO3)2 content used in thermal shock method on the photocatalytic activity of Cu-Modified ZnO nanoparticles

The Cu doped ZnO photocatalysts were prepared on ZnO substrate modified with copper nitrate by thermal shock method with different ratio % molar Cu : Zn = 0.3, 0.5, 1.0, 2.0 and 5.0 in order to study the impacts of copper content on the photocatalytic activity of ZnO under both UV and Vis light irradiation. The crystal structure, morphology bulk and surface were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Their photocatalytic activities were studied via time-dependent degradation of methylene blue in aqueous solution. The results exhibit that crystal structure and morphology of Cu doped ZnO photocatalysts is not modified significally than ZnO original but surface charateristicschanged greatly. The photocatalyst was doped with copper content under 2% showed formation of Cu species. These samples perform photocatalytic activity higher than ZnO. The CuNZO-0.05-500 had the highest rate constants for methylene blue degradation (kUV = 6,901 h-1, kVIS = 0,224 h-1), which are about 2.2 times and 1.3 times higher than unmodified ZnO under UV light and Vis light, respectively. However, the CuNZO-5.0-500 which had the formation of CuO phase and unchangeable ZnO's surface has photocatalytic activity similar to pure ZnO.