ZnO Single Crystals Research Articles

Advanced Nested Coaxial Thin-Film ZnO Nanostructures Synthesized by Atomic Layer Deposition for Improved Sensing Performance

We report a new synthesis method for multiple-walled nested thin-film nanostructures by combining hydrothermal growth methods with atomic layer deposition (ALD) thin-film technology and sacrificial films, thereby increasing the surface-to-volume ratio to improve the sensing performance of novel ZnO gas sensors. Single-crystal ZnO nanorods serve as the core of the nanostructure assembly and were synthesized hydrothermally on fine-grained ALD ZnO seed films. Subsequently, the ZnO core nanotubes were coated with alternating sacrificial coaxial 3D wrap-around ALD Al2O3 films and ALD ZnO films. Basically, the center nanorod was coated with an ALD 3D wrap-around Al2O3 sacrificial layer to realize a nested coaxial ZnO thin-film nanotube. To increase the surface-to-volume ratio of the nested multiple-film nanostructure, both the front and backside of the nested coaxial ZnO films must be exposed by selectively removing the intermittent Al2O3 sacrificial films. The selective removal of the sacrificial films exposes the front and backside of the free-standing ZnO films for interaction with target gases during sensing operation while steadily increasing the surface-to-volume ratio. The sensing response of the novel ZnO gas sensor architecture with nested nanotubes achieved a maximum 150% enhancement at low temperature compared to a conventional ZnO nanorod sensor.

Microbial biosurfactant-mediated green synthesis of zinc oxide nanoparticles (ZnO NPs) and exploring their role in enhancing chickpea and rice seed germination

Malnutrition is one of the greatest challenges faced by humanity, which may be addressed by improving crop productivity to ensure food security. However, extensive use of synthetic fertilizers can lead to soil fertility degradation. This study highlights the potential of combining nanotechnology with biotechnology to enhance the germination rates of commercially important crop seeds. Bacterial biosurfactant extracted from a newly isolated Klebsiella sp. strain RGUDBI03 was used as a reducing and capping agent for the synthesis of zinc oxide nanoparticles (ZnO NPs) through a simple method. Extensive characterization of ZnO NPs through electron microscopic analysis showed well-dispersed, homogeneous NPs with a size range of 2–10 nm. High-resolution transmission electron microscopy (HR-TEM) images also revealed molecular fringes of 0.26 nm in single crystal ZnO NPs, with approximately 50% of the NPs exhibiting a size range of 2–4 nm. X-ray diffraction (XRD) results of ZnO NPs indicated the presence of (100), (002), (101), (102), (200), and (112) planes, confirming their crystalline nature. The presence of C = C–H, C = C, C–H, and C = C groups in both the bacterial biosurfactant and ZnO NPs, as depicted by Fourier-transform infrared spectroscopy (FTIR) spectra, confirmed the function of the biosurfactant as a reducing and capping agent. The nano-primed chickpea (Cicer arietinum) and rice (Oryza sativa) seeds showed an increase in water uptake rate, 89% and 92% respectively, compared to the control (73% and 44%), leading to an enhanced germination rate of 98% and 76%, compared to their respective controls (80% and 30%) under optimized conditions. Additionally, the nano-primed seeds exhibited higher levels of α-amylase activity in both seeds (0.37 mg/g for chickpea and 2.49 mg/g for rice) compared to the control. Notably, the ZnO NP priming solution exhibited no cytotoxicity on red blood cells and earthworms (Eudrilus eugeniae), indicating their non-cytotoxic and eco-friendly nature for future field trials.

Enhanced emission and surface modification in ZnO treated with Ar, H, and O plasmas

Plasma treatment is a crucial technique for manipulating the optoelectronic properties of ZnO devices by modifying surface defects. The choice of plasma gas in the treatment process significantly influences these defects, as evidenced by the luminescence characteristics. In this paper, significant differences have been observed in the photoluminescence spectra of ZnO films and single crystals before and after treatment with three kinds of plasma gas. After Ar and H plasma treatment, the UV emission is significantly enhanced up to 1000 times, while the visible emission is suppressed. Conversely, after O plasma treatment, the UV emission remains unchanged, but the deep level emission exhibits a subtle transformation. Since the influence depth of plasma treatment is confined to the surface, all these alterations of typical luminescence are strongly dependent on surface-related defects, thereby challenging the previous interpretation that they originate from bulk defects. The modification effects of various plasmas on surfaces are confirmed from multiple perspectives, and the mechanisms behind the enhancement or suppression of luminescence are systematically analyzed. A comprehensive understanding of plasma treatment, along with precise control of surface defects, is crucial for optimizing the performance of ZnO devices.

Unconventional exchange bias and enhanced spin pumping efficiency due to diluted magnetic oxide at the Co/ZnO interface

Exchange bias (EB) is normally created by the interfacial exchange coupling at a ferromagnetic/antiferromagnetic (FM/AFM) interface. FM/AFM interfaces have also been proved to perform enhanced spin angular momentum transfer efficiency in spin pumping (SP), compared with typical FM/nonmagnetic interfaces. Here, we report an unexpected EB and enhanced SP between a ferromagnet and semiconductor. Considerable EB has been observed in Co films grown on ZnO single crystal due to the interface antiferromagnetism of the Zn1−xCoxO (x depends on the Co solubility limit in ZnO) layer. Moreover, SP measurements demonstrate a giant spin pumping efficiency at the Co/ZnO interface with a bump (spin mixing conductance Geff↑↓= 28 nm−2) around the blocking temperature TB ∼ 75 K. The enhanced SP is further confirmed by inverse spin Hall effect measurements and the spin Hall angle θISHE of Zn1−xCoxO is estimated to be 0.011. The bound magnetic polarons with s–d exchange interaction between donor electrons and magnetic cation ions in Zn1−xCoxO play a key role in the formation of antiferromagnetism with giant Geff↑↓. Our work provides a new insight into spin physics at FM/semiconducting interfaces.

Manipulating four-photon absorption of ZnO via Ga doping.

Multi-photon absorption in the second near-infrared (NIR-II) regime has attracted extensive attention due to biological imaging and frequency-upconverted lasing applications. We report the dispersion of four-photon absorption (4 PA) response in pristine and Ga-doped ZnO single crystals over the spectral range 1180-1350 nm. Femtosecond Z-scan results demonstrate that Ga doping can significantly enhance the 4 PA coefficient β4 of ZnO. Interestingly, the wavelength dependency of β4 in Ga-doped ZnO shows a strong resonance around 1215-1250 nm, which is correlated with the PL peak of Ga-doped ZnO at 405 nm. Femtosecond pump-probe measurements validate that Ga doping has no profound impact on the ultrafast carrier relaxation of ZnO, indicating Ga doping leads to a shallow state rather than a deep trap within the bandgap. The possible mechanism of 4 PA enhancement induced by degeneracy with multi-photon absorption resonance to the Ga-doped state is discussed. Our results verify the strong potential of Ga-doped ZnO with tunable nonlinear optical properties as a promising candidate for nonlinear optical and nanophotonic devices in the NIR-II region.

Readout circuit for a ZnO bulk-acoustic-wave X-ray dose rate detector

Bulk Acoustic Wave (BAW) resonators based on piezoelectric semiconductors are radio frequency devices that can convert X-ray dose rate into resonant frequency offset, which exhibit strong anti-interference capabilities and low power consumption and are suitable for wireless sensing systems. In this study, we designed a readout circuit for a ZnO BAW X-ray dose rate detector. (0001)-oriented ZnO single crystals were used to fabricate the high-quality BAW X-ray detector. Under dark field and X-ray irradiation conditions, the resonant frequency offset of the detector was recorded via the readout circuit. The readout circuit consists of an oscillator circuit based on a Pierce oscillator circuit structure, a frequency mixing circuit, and an STM32 microcontroller. This design offers higher frequency stability and lower phase noise. In practical applications, it can replace the expensive and bulky vector network analyzer for detecting and storing the resonant frequency of BAW resonators with the advantages of fast readout speed and portability.

Observation of the Photoenhanced Piezoelectric Effect on (101̅0) Orientated ZnO Single Crystal

Observation of the Photoenhanced Piezoelectric Effect on (101̅0) Orientated ZnO Single Crystal

Significance of the Different Exposed Surfaces of ZnO Single Crystals and Nanowires on the Photocatalytic Activity and Processes

The heterogeneous photocatalysis of organic dyes using ZnO nanowires (NWs) is of high interest to face the challenge of eco‐efficient water remediation. However, the effects of the wurtzite structure of ZnO and hence of the shape of nanostructures on the photocatalytic processes are still under debate. Herein, it is shown that the photocatalytic activity of ZnO single crystals with five different orientations follows a pseudo‐first‐order kinetics as: () < {} < {} < {} < (). The photocatalytic processes are independent of the nature of the crystallographic planes, apart from the semipolar {} orientation. Interestingly, ZnO NWs exhibit a photocatalytic activity that is relatively independent of their length, which is neither due to the penetration of organic dyes nor to the penetration of UV light. Instead, the sidewalls of ZnO NWs are much less efficient than the ZnO single crystal with the same nonpolar m‐plane orientation, indicating that the structural morphology and chemical composition of the surface, as well as their much higher doping level, govern the photocatalytic activity and processes. These findings indicate that the increase in the photocatalytic activity of ZnO NWs should be addressed by designing more active surfaces rather than simply increasing their total surface area.

Transient positron annihilation spectroscopy under light irradiation at liquid nitrogen temperature: evaluation of defect states in single-crystal ZnO

Abstract A time-dependent measurement system has been developed for positron annihilation spectroscopy to study the effects of light irradiation at liquid nitrogen temperature. The system enables the measurement of positron annihilation lifetimes after pulse light irradiation, utilizing various time windows to investigate the transient changes of photo-excited vacancy-type defects. Additionally, this system facilitates coincidence Doppler broadening measurements during light irradiation at liquid nitrogen temperature. The system was successfully employed to analyze changes in positron lifetime and positron annihilation sites in electron-irradiated single-crystalline ZnO under light irradiation.

Analysis of the Effects of Parameters on the Performance of Resonators Based on a ZnO/SiO2/Diamond Structure

With the development of communications technology, surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices have become hotspots of the competitive research in the frequency band above GHz. It imposes higher requirements on the operating frequency, temperature coefficient of frequency (TCF), and electromechanical coupling coefficient (k2) of SAW devices. In this work, we reported on a novel ZnO/SiO2/diamond-layered resonator structure and systematically investigated its propagation characteristics by using finite element methods. A comparative study and analysis of k2 and acoustic velocity (vp) for both the excited Rayleigh mode and the Sezawa mode were conducted. By selecting the appropriate ZnO piezoelectric film, SiO2, and electrode thickness, the Sezawa mode was chosen as the main mode, effectively improving both k2 and vp. It was observed that the k2 of the Sezawa mode is 7.5 times that of the excited Rayleigh mode and nearly 5 times that of piezoelectric single-crystal ZnO; vp is 1.7 times that of the excited Rayleigh mode and nearly 1.5 times that of piezoelectric single-crystal ZnO. Furthermore, the proposed multilayer structure achieves a TCF close to 0 while maintaining a substantial k2. In practical applications, increasing the thickness of SiO2 can compensate for the device’s TCF reduction caused by the interdigital transducer (IDT). Finally, this study explored the impact of increasing the aperture width and IDT pairs on the performance of the single-port resonator, revealing the changing patterns of quality factor (Q) values. The results reported here show that the structure has great promise for the fabrication of high-frequency and low-TCF SAW devices.

Scintillation properties of (Ph4P)2ZnX4 (X = Cl, Br)

We fabricated (Ph4P)2ZnCl4 and (Ph4P)2ZnBr4 crystals (Ph4P: tetraphenylphosphonium) with zero-dimensional structures via solvent evaporation and evaluated their optical and scintillation properties. Fluorescence and phosphorescence peaks derived from Ph4P+ cations appeared at 345 and 510 nm, respectively, for both crystals, while an emission originating from self-trapped excitons was also detected for (Ph4P)2ZnBr4. In the scintillation spectra, both samples exhibited a phosphorescence peak, while a weak fluorescence peak was also observed for (Ph4P)2ZnCl4. In addition, the scintillation light yield (LY) was determined by pulse-height spectra with 1 μs shaping time. A pulse-height peak was observed from (Ph4P)2ZnCl4, and the scintillation LY was about 670 photons/5.5 MeV-α, which was higher than that of a ZnO single crystal.

Recombination dynamics and pronounced re-absorption effect in ZnO single crystals under two-photon excitation

Time-resolved photoluminescence measurements were conducted on ZnO single crystals using two-photon (2P) excitation at low temperatures ranging from 25 to 260 K. The decay constants of various emission peaks are determined and compared between one-photon and two-photon excitations using a recently modified localized-state ensemble model. The analysis is focused on the redshift of the phonon-assisted free exciton transition energy with increasing temperature. A significantly longer radiative recombination lifetime of approximately 840 ps is observed under 2P excitation. The extended lifetime of excitons during 2P excitation is achieved through the interplay of multiple mechanisms. Photon-recycling, enabled by high absorption coefficients and total internal reflection, facilitates re-absorption and new carrier generation. Phonon scattering lowers photon energy, allowing their escape from the crystal, while exciton–polariton interactions delay photon travel, collectively contributing to the extension of exciton lifetimes. The re-absorption effect and elongated carrier lifetime observed from our work could be beneficial for applications of ZnO materials in 2P imaging, particularly of biological samples.

Polishing and etching damages of ZnO single crystals studied using time-resolved photoluminescence spectroscopy

Controlled thinning of wide bandgap semiconductors by chemo-mechanical polishing (CMP) and/or reactive ion etching (RIE) has been one of the versatile methods for various optoelectronic applications. The influences of CMP and subsequent wet chemical etching, as well as independent RIE, on the room-temperature photoluminescence lifetime for the near-band-edge emission [τPLNBE(RT)] of O-polarity c-plane ZnO single crystals were examined by using time-resolved photoluminescence measurements. τPLNBE(RT) decreased from a nanosecond range to a few picoseconds (ps) by a conventional CMP, indicating a generation of high-concentration midgap recombination centers, such as nonradiative recombination centers and deep radiative recombination centers. τPLNBE(RT) was progressively regained up to 600 ps by a subsequent etching using HCl aqueous solution. However, the recovery saturated at the etching depth of about 200 nm and τPLNBE(RT) was not restored even after etching by 350 nm. The results indicate the introduction of certain structural deformations during the CMP. Because x-ray diffraction measurement revealed the presence of incoherent surface domains right after the CMP and the HCl etching gave rise to inhomogeneously etched canyons, nonradiative recombination centers, such as dislocations and vacancy clusters, are likely generated by mechanical shear stresses. τPLNBE(RT) also decreased by the RIE. However, the degradation was less significant than the case for the CMP, because RIE scarcely gives mechanical stresses. Interestingly, τPLNBE(RT) for the samples etched under higher plasma power was longer than the lower power cases. From the results of x-ray photoelectron spectroscopy measurements, unintentionally deposited oxide films containing Si are proposed to act as an attenuating layer for the introduction of nonradiative recombination centers.

Unraveling Anisotropic and Pulsating Etching of ZnO Nanorods in Hydrochloric Acid via Correlative Electron Microscopy.

Despite much technical progress achieved so far, the exact surface and shape evolution during wet chemical etching is less unraveled, especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, a repeated two-stage anisotropic and pulsating periodic etching dynamic is discovered during the pencil shape evolution of a single crystal ZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopencil tip shrinks at a slower rate along [0001̅] than that along the ⟨101̅0⟩ directions, resulting in a sharper ZnO pencil tip. Afterward, rapid tip dissolution happens due to accelerated etching rates along various crystal directions. Concurrently, the vicinal base region of the original nanopencil tip emerges as a new tip followed by the repeated sequence of tip shrinking and removal. The high-index surfaces, such as {101̅m} (m = 0, 1, 2, or 3) and {21̅ 1̅n} (n = 0, 1, 2, or 3), are found to preferentially expose in different ratios. Our 3D electron tomography, convergent beam electron diffraction, middle-angle bright-field STEM, and XPS results indicate the dissociative Cl- species were bound to the Zn-terminated tip surfaces. Furthermore, DFT calculation suggests the preferential Cl- passivation over the {101̅1} and (0001) surfaces of lower energy than others, leading to preferential surface exposures and the oscillatory variation of different facet etching rates. The boosted reactivity due to high-index nanoscale surface exposures is confirmed by comparatively enhanced chemical sensing and CO2 hydrogenation activity. These findings provide an in-depth understanding of anisotropic wet chemical etching of ionic nanocrystals and offer a design strategy for advanced functional materials.

Negative temperature coefficient of ZnO microwires for cryogenic temperature sensing

Negative temperature coefficient (NTC) thermistors are expected to be developed at cryogenic temperature sensing. In this paper, a kind of cryogenic thermistor was developed based on NTC response of single crystal ZnO microwires (MWs). The current–voltage (I–V) characterization demonstrated a NTC response, which was temperature dependence of resistance that decreased with increasing temperature. A sensitive NTC response, especially at the cryogenic temperature range, was observed. The defects in ZnO MWs play an important role in NTC response; therefore, we studied the origin of NTC effect associated with defects to improve the recognition of relationship between defects and the NTC effect and further optimize the temperature sensor design for high performance. Annealing at 800 °C in air was undertaken to make a significant influence on the concentration of defects in as-grown sample, and a series of temperature-dependent features were investigated by photoluminescence, Raman, XPS, and EPR measurements. The results indicated the zinc interstitials to be effective donors for sensitive NTC effect at the cryogenic region. This study provides insight into the ZnO NTC effect and sensitive cryogenic sensing technology.

Optical and scintillation properties of organic–inorganic perovskite-type compounds with various hydroxyl alkyl amines or alkyl ether amines

We evaluated photoluminescence and radiation response properties of organic–inorganic perovskite-type compounds with various hydroxyl alkyl amines or alkyl ether amines (HOCnH2nNH3)2PbCl4 (n = 2: C2–OH, n = 3: C3–OH, n = 4: C4–OH), (CH3CH2OCnH2nNH3)2PbCl4 (n = 2: 2-Ethoxy, n = 3: 3-Ethoxy), and (CH3C2H4OC3H6NH3)2PbCl4 (3-Butoxy). A clear broadband emission peak attributed to self-trapped excitons (STEs) was detected from the C2–OH, C3–OH, and 2-Ethoxy crystals under 310 nm excitation light. Additionally, they showed a fast decay time of 3.5 ns (C2–OH), 5.5 ns (C3–OH), and 3.0 ns (2-Ethoxy), which was ascribed to the recombination of STEs. In the scintillation spectra, the C2–OH, C3–OH, and 2-Ethoxy crystals exhibited a broad emission originating from the recombination to STEs. Furthermore, they exhibited a fast scintillation decay time of 2.2 ns (C2–OH), 3.5 ns (C3–OH), and 1.1 ns (2-Ethoxy) due to the recombination of STEs. Moreover, the light yield of the C3–OH crystal under alpha-ray was the highest, and the yield was estimated to be 1100 photons/5.5 MeV-α, which was higher than that of a ZnO single crystal.

Single-crystal ZnO microstructures for improved triethylamine-sensing performance

Single-crystal ZnO microstructures for improved triethylamine-sensing performance

Shedding new light on the dislocation-mediated plasticity in wurtzite ZnO single crystals by photoindentation

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness.

Optical characterization of high-quality ZnO (0002) / Cu (111) epilayers grown by electrodeposition

Wurtzite ZnO thin films were grown on Cu substrates of three orientations – (111), (100), and (130), respectively – by a simple electrodeposition technique. Among them, the films deposited on the (111) and (130) substrates were epilayers with different crystallinity. Optical characterizations were therefore conducted for the (0002)ZnO/(111)Cu epilayers showing promising crystallinity while having different morphologies. Photoluminescence (PL) spectra of epilayers show a sharp UV near band edge (NBE) emission band, owing to excitonic transitions in ZnO layer, whereas a wide feature in visible spectral region is related to deep defect (O-vacancy) states in ZnO. The strong excitonic ZnO band (NBE) suppresses the defect (DEF) band intensity by ∼100 times, indicating good optical quality of ZnO epilayers. Using the Arrhenius-type equation, activation energies were determined as Ea = 23 meV and 27 meV, which appeared to be related with the Stokes shift between emission and absorption spectra. From the excitation-dependent (80–700 kW/cm2) PL measurements at 3 K, a linear behavior of ratio NBE:DEF was deduced, which indicated that excitonic- and defect-related radiative recombination paths are independent. Furthermore, from the dependence of the PL intensity on the laser power density, characterized with exponential factor of k ≃ 1.4, the donor-bound nature of the excitonic transitions (DoX) in the ZnO/Cu epilayers is suggested. Corroborative structural and optical findings with the bandgap values estimated (3.362 eV and 3.371 eV) in a very good agreement with the literature data for ZnO single crystal, confirm high-quality of the studied ZnO/Cu (111) epilayers.

In-Plane Anisotropic Rectification and Photovoltaic Effects on ZnO Nonpolar (101̅0) Crystal Plane and the Physical Mechanism.

The concept of spontaneous electric field (Es) in polar structures is crucial for understanding the physical and chemical properties of compound semiconductors and improving their performanes. However, this concept has not been widely accepted so far. Here, we show the first observation of rectification and photovoltaic effects in the polar [0001] direction on the nonpolar ZnO (1010) crystal plane. However, no rectification and photovoltaic effects are observed in the nonpolar [1210] direction perpendicular to the [0001]. When a stress was applied in the [0001] direction of the ZnO single crystal, the rectification and photovoltaic effects are abated and disappeared. The disappearance of the two effects results from the pressure-induced disappearance of the polar structure. The results fully demonstrated that the rectification and photovoltaic effects arise from the existence of Es in the polar [0001] direction. The Es motivates the directional transfer of the electrons and photocreated charges along the [0001] direction, and the rectification and photovoltaic effects are thus observed. These results provide direct evidence for the polar structure theory and suggest that the polar structures can be employed to develop new types of photovoltaic and other electronic and photoelectronic devices.