Band Alignment Engineering at Cu2O/ZnO Heterointerfaces

Electro-Thermo-Magnetic Effect-Induced Large Thermoelectric Performance of Calcium Cobaltite Nanocomposites

As attractive thermoelectric oxides, Ca3Co4O9-based materials have been intensively studied for their applications in recent years. However, their thermoelectric performance is enormously limited due to the contradiction of electrical resistivity and thermal conductivity. Herein, BaFe12O19 nanospheres were introduced into the Ca3Co4O9 matrix. The metallic Ag, ferrites, and matrix phase survived together, and a high density of nanoscale BaFe12O19 precipitation was observed. The reduction of work function could lead to band bending and form an interface potential due to the electro-thermo-magnetic effect contributing to the hole migration. As a result, a huge ZT value of 0.51 for the 8 wt % BaFe12O19/Ca3Co4O9 nanocomposites was obtained at 1073 K, accompanied by a low electrical resistivity of 6.7 mΩ·cm and a high Seebeck coefficient of 217.5 μV/K. In addition, a significant reduction of thermal conductivity (1.11 W/(m·K)) occurred, which was due to the nanoscale ferromagnetic phase effectively scattering the mid- and short-wavelength heat-carrying phonons. The synergistic enhancement of thermoelectric performance confirmed that the electro-thermo-magnetic effect is an effective way to solve the challenging problem of performance deterioration in oxide thermoelectric materials.

Improved Ultrafast Carrier Relaxation and Charge Transfer Dynamics in CuI Films and Their Heterojunctions via Sn Doping

CuI is one of the promising hole transport materials for perovskite solar cells. However, its tendency to form defects is currently limiting its use for device applications. Here, we report the successful improvement of CuI through Sn doping and the direct measurement of the carrier relaxation and interfacial charge-transfer processes in Sn-doped CuI films and their heterostructures. Femtosecond-transient absorption (fs-TA) measurements reveal that Sn doping effectively passivates the trap states within the bandgap of CuI. The I-V characteristics of heterostructures demonstrate drastic improvement in transport characteristics upon Sn doping. Fs-TA measurements further confirm that the CuSnI/ZnO heterojunction has a type-II configuration with ultrafast charge transfer (<280 fs). The charge transfer time of a CuI/ZnO heterostructure is ∼2.8 times slower than that of the CuSnI/ZnO heterostructure, indicating that Sn doping suppresses the interfacial states that retard the charge transfer. These results elucidate the effect of Sn doping on the performance of CuI-based heterostructures.

Dopant-Tunable Ultrathin Transparent Conductive Oxides for Efficient Energy Conversion Devices

Ultrathin film-based transparent conductive oxides (TCOs) with a broad work function (WF) tunability are highly demanded for efficient energy conversion devices. However, reducing the film thickness below 50 nm is limited due to rapidly increasing resistance; furthermore, introducing dopants into TCOs such as indium tin oxide (ITO) to reduce the resistance decreases the transparency due to a trade-off between the two quantities. Herein, we demonstrate dopant-tunable ultrathin (≤ 50 nm) TCOs fabricated via electric field-driven metal implantation (m-TCOs; m = Ni, Ag, and Cu) without compromising their innate electrical and optical properties. The m-TCOs exhibit a broad WF variation (0.97 eV), high transmittance in the UV to visible range (89-93% at 365 nm), and low sheet resistance (30-60 Ω cm^-2). Experimental and theoretical analyses show that interstitial metal atoms mainly affect the change in the WF without substantial losses in optical transparency. The m-ITOs are employed as anode or cathode electrodes for organic light-emitting diodes (LEDs), inorganic UV LEDs, and organic photovoltaics for their universal use, leading to outstanding performances, even without hole injection layer for OLED through the WF-tailored Ni-ITO. These results verify the proposed m-TCOs enable effective carrier transport and light extraction beyond the limits of traditional TCOs.

Copper-zinc oxide heterojunction catalysts exhibiting enhanced photocatalytic activity prepared by a hybrid deposition method

Heterojunction copper-zinc oxide catalysts were prepared by a hybrid two-step methodology comprising hydrothermal growth of ZnO nanorods (ZnO-NR) followed by deposition of Cu2O nanoparticles using an advanced gas deposition technique (AGD). The obtained bicatalysts were characterized by SEM, AFM, XRD, XPS, PL and spectrophotometry and revealed well-dispersed and crystalline Cu2O nanoparticles attached to the ZnO-NR. The adsorption properties and photocatalytic degradation of Orange II dye in water solutions were measured. It was found that the bicatalysts exhibited a conversion rate and quantum yield that both were about 50% higher compared with ZnO-NR alone, which were attributed to the intrinsic electric field created at the p-n junction formed at the Cu2O/ZnO interface facilitating charge separation of electron-hole pairs formed upon interband photon absorption. The interpretation was evidenced by efficient quenching of characteristic deep level ZnO photoluminescence bands and photoelectron core-level energy shifts. By comparisons with known energy levels in Cu2O and ZnO, the effect was found to be most pronounced for the non-polar ZnO-NR side facets, which accounted for about 95% of the exposed surface area of the catalyst and hence the majority of dye adsorption. It was also found that the dye adsorption capacity of the ZnO nanorods increased considerably after Cu2O deposition thereby facilitating the oxidation of the dye. The results imply the possibility of judiciously aligning band edges on structurally controlled and well-connected low-dimensional semiconductor nanostructures using combined two-step synthesis techniques, where in particular vacuum-based techniques such as AGD allow for growth of well-connected nanocrystals with well developed heterojunction interfaces.

Continuously Improved Photocatalytic Performance of Zn2SnO4/SnO2/Cu2O Composites by Structural Modulation and Band Alignment Modification

Improving the photocatalytic performance of multi-component photocatalysts through structural modulation and band alignment engineering has attracted great interest in the context of solar energy utilization and conversion. In our work, Zn₂SnO₄/SnO₂ hierarchical architectures comprising nanorod building block assemblies were first achieved via a facile solvothermal synthesis route with lysine and ethylenediamine (EDA) as directing agents, and then chemically etched in NaOH solution to enlarge the surface area and augment active sites. The etched Zn₂SnO₄/SnO₂ hierarchical architectures were further decorated by Cu₂O nanoparticles though an in situ chemical deposition method based on band alignment engineering. In comparison with unetched Zn₂SnO₄/SnO₂, the specific surface area of Zn₂SnO₄/SnO₂/Cu₂O hierarchical architectures became larger, and the responsive region and absorbance intensity became wider and higher in the whole visible-light range. Zn₂SnO₄/SnO₂/Cu₂O hybrid photocatalysts presented enormously improved visible-light photocatalytic behaviour for Rhodamine B (RhB) decomposition. The enhancement of photocatalytic behaviour was dominantly attributed to the synergy effect of the larger specific surface area, higher light absorption capacity, and more effective photo-induced charge carrier separation and migration. A proposed mechanism for the enormously promoted photocatalytic behaviour is brought forth on the basis of the energy-band structure combined with experimental results.

Electrical Properties of Low-Temperature Processed Sn-Doped In2O3 Thin Films: The Role of Microstructure and Oxygen Content and the Potential of Defect Modulation Doping

Low-temperature-processed ITO thin films offer the potential of overcoming the doping limit by suppressing the equilibrium of compensating oxygen interstitial defects. To elucidate this potential, electrical properties of Sn-doped In2O3 (ITO) thin films are studied in dependence on film thickness. In-operando conductivity and Hall effect measurements during annealing of room-temperature-deposited films, together with different film thickness in different environments, allow to discriminate between the effects of crystallization, grain growth, donor activation and oxygen diffusion on carrier concentrations and mobilities. At 200∘C, a control of carrier concentration by oxygen incorporation or extraction is only dominant for very thin films. The electrical properties of thicker films deposited at room temperature are mostly affected by the grain size. The remaining diffusivity of compensating oxygen defects at 200∘C is sufficient to screen the high Fermi level induced by deposition of Al2O3 using atomic layer deposition (ALD), which disables the use of defect modulation doping at this temperature. The results indicate that achieving higher carrier concentrations in ITO thin films requires a control of the oxygen pressure during deposition in combination with seed layers to enhance crystallinity or the use of near room temperature ALD.

Heterointerface-Driven Band Alignment Engineering and its Impact on Macro-Performance in Semiconductor Multilayer Nanostructures

Interfaces in semiconductor heterostructures is of continuously greater significance in the trend of scaling materials down to the atomic limit. Since atoms tend to behave more irregularly around interfaces than in internal materials, accurate energy band alignment becomes a major challenge, which determines the ultimate performance of devices. Therefore, a comprehensive understanding of the interplay between heterointerface, energy band, and macro-performance is desiderated. Here, such interplay is explored by investigating asymmetric heterointerfaces with identical fabrication parameters in multiple-quantum-well lasers. The unexpected asymmetry derives from the atomic discrepancy around heterointerfaces, which ultimately improves the optical property through altered valence band offsets. Strain and charge distribution around heterointerfaces are characterized via geometric phase analysis and in situ bias electron holography, respectively. Combining experiments with theories, arsenic-enrichment at one of the interfaces is considered the origin of asymmetry. To reveal actual band alignment, valence band model is modified focusing on the transition around heterojunctions. The enhanced photoluminescence intensity reflects the alleviation of hole confinement insufficiency and the enlargement of valence band offset. The results help to advance the understanding of the general problem of interface in nanostructures and provide guidance applicable to various scenarios for micro-macro correlation.

Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes

Cuprous oxide (Cu₂O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu₂O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu₂O surface as well as the interface between Cu₂O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu₂O and provide guidance for future material development.

While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. Here, the authors use femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses.

Use of a New Non-Pyrophoric Liquid Aluminum Precursor for Atomic Layer Deposition

An Al₂O₃ thin film has been grown by vapor deposition using different Al precursors. The most commonly used precursor is trimethylaluminum, which is highly reactive and pyrophoric. In the purpose of searching for a more ideal Al source, the non-pyrophoric aluminum tri-sec-butoxide ([Al(O^sBu)₃], ATSB) was introduced as a novel precursor for atomic layer deposition (ALD). After demonstrating the deposition of Al₂O₃ via chemical vapor deposition (CVD) and ‘pulsed CVD’ routes, the use of ATSB in an atomic layer deposition (ALD)-like process was investigated and optimized to achieve self-limiting growth. The films were characterized using spectral reflectance, ellipsometry and UV-Vis before their composition was studied. The growth rate of Al₂O₃ via the ALD-like process was consistently 0.12 nm/cycle on glass, silicon and quartz substrates under the optimized conditions. Scanning electron microscopy and transmission electron microscopy images of the ALD-deposited Al₂O₃ films deposited on complex nanostructures demonstrated the conformity, uniformity and good thickness control of these films, suggesting a potential of being used as the protection layer in photoelectrochemical water splitting.

Toward Robust Photoelectrochemical Operation of Cuprous Oxide Nanowire Photocathodes Using a Strategically Designed Solution-Processed Titanium Oxide Passivation Coating

To date, TiO₂ films prepared by atomic layer deposition are widely used to prepare Cu₂O nanowire (NW)-based photocathodes with photoelectrochemical (PEC) durability as this approach enables conformal coating and furnishes chemical robustness. However, this common approach requires complicated interlayers and makes the fabrication of photocathodes with reproducible performance and long-term stability difficult. Although sol-gel-based approaches have been well established for coating surfaces with oxide thin films, these techniques have rarely been studied for oxide passivation in PEC applications, because the sol-gel coating methods are strongly influenced by surface chemical bonding and have been mainly demonstrated on flat substrates. As a unique strategy based on solution processing, herein, we suggest a creative solution for two problems encountered in the conformal coating of surfaces with oxide layers: (i) how to effectively prevent corrosion of materials with hydrophilic surfaces by simply using a single TiO₂ surface protection layer instead of a complex multilayer structure and (ii) guaranteeing perfect chemical durability. A Cu(OH)₂ NW can be easily prepared as an intermediate phase by anodization of a Cu metal, where the former inherently possesses a hydrophilic hydroxylated surface and thus, enables thorough coating with TiO₂ precursor solutions. Chemically robust nanowires are then generated as the final product via the phase transformation of Cu(OH)₂ to Cu₂O via sintering at 600 °C. The coated NWs exhibit excellent PEC properties and a stable performance. Consequently, the perfect chemical isolation of the Cu₂O NWs from the electrolyte allows a remarkable PEC operation with the maintenance of the initial photocurrent for more than one day.

Metal Oxide Thin-Film Heterojunctions for Photovoltaic Applications

Silicon-based tandem solar cells incorporating low-cost, abundant, and non-toxic metal oxide materials can increase the conversion efficiency of silicon solar cells beyond their conventional limitations with obvious economic and environmental benefits. In this work, the electrical characteristics of a metal oxide thin-film heterojunction solar cell based on a cuprous oxide (Cu₂O) absorber layer were investigated. Highly Al-doped n-type ZnO (AZO) and undoped p-type Cu₂O thin films were prepared on quartz substrates by magnetron sputter deposition. The electrical and optical properties of these thin films were determined from Hall effect measurements and spectroscopic ellipsometry. After annealing the Cu₂O film at 900 °C, the majority carrier (hole) mobility and the resistivity were measured at 50 cm²/V·s and 200 Ω·cm, respectively. Numerical modeling was carried out to investigate the effect of band alignment and interface defects on the electrical characteristics of the AZO/Cu₂O heterojunction. The analysis suggests that the incorporation of a buffer layer can enhance the performance of the heterojunction solar cell as a result of reduced conduction band offset.

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron

Cupric oxide (CuO) is considered as a promising photocathode material for photo(electro)chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core-shell structure of the nanowires.

Bond-Energy-Driven, Low- or High-Angle-Grain-Boundary-Movement-Mediated Synthesis of Porous Se-Te for Use in Water-Splitting Reactions

Herein, for the first time, we applied the metal-metal-bond-energy factor to the evolution of a porous Se-Te alloy. The porous Se-Te material has been prepared from the constituents' elemental states, through only a heating-cooling process in silicone oil without the use of any reagent, surfactant, or capping agent. Surprisingly, the reaction occurred at a much lower temperature (240 °C) than the mp (450 °C) of Te⁰. The reaction's nucleation and growth by means of varied bond energy have been clarified for the first time. A difference in the bond energies of a hetero metal-metal bond (Se-Te) and a homo metal-metal bond (Se-Se) directs nucleation and growth toward the fabrication of a porous structure, even from the constituents' elemental states, in which low-angle-grain-boundary (LAGB) and high-angle-grain-boundary (HAGB) movements play governing roles. Proper band-gap alignment of Se and Te makes the alloy composite applicable to water-splitting reactions under Xe-arc-lamp illumination. PEC efficiency of Se-Te was found to be higher than those reported for Se and other composite materials.

Hydrothermal synthesis of In2O3 nanoparticles hybrid twins hexagonal disk ZnO heterostructures for enhanced photocatalytic activities and stability

In₂O₃ nanoparticles hybrid twins hexagonal disk (THD) ZnO with different ratios were fabricated by a hydrothermal method. The as-obtained ZnO/In₂O₃ composites are constituted by hexagonal disks ZnO with diameters of about 1 μm and In₂O₃ nanoparticles with sizes of about 20-50 nm. With the increase of In₂O₃ content in ZnO/In₂O₃ composites, the absorption band edges of samples shifted from UV to visible light region. Compared with pure ZnO, the ZnO/In₂O₃ composites show enhanced photocatalytic activities for degradation of methyl orange (MO) and 4-nitrophenol (4-NP) under solar light irradiation. Due to suitable alignment of their energy band-gap structure of the In₂O₃ and ZnO, the formation of type п heterostructure can enhance efficient separation of photo-generate electro-hole pairs and provides convenient carrier transfer paths.

Band-Bending of Ga-Polar GaN Interfaced with Al2O3 through Ultraviolet/Ozone Treatment

Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O₃) treatment of the GaN surface on the energy band bending of atomic-layer-deposition (ALD) Al₂O₃ coated Ga-polar GaN were studied. The UV/O₃ treatment and post-ALD anneal can be used to effectively vary the band bending, the valence band offset, conduction band offset, and the interface dipole at the Al₂O₃/GaN interfaces. The UV/O₃ treatment increases the surface energy of the Ga-polar GaN, improves the uniformity of Al₂O₃ deposition, and changes the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O₃ treatment-induced surface factors externally screen the effect of polarization charges in the GaN, in effect, determining the eventual energy band bending at the Al₂O₃/GaN interfaces. An optimal UV/O₃ treatment condition also exists for realizing the "best" interface conditions. The study of UV/O₃ treatment effect on the band alignments at the dielectric/III-nitride interfaces will be valuable for applications of transistors, light-emitting diodes, and photovoltaics.

Band Alignment at GaN/Single-Layer WSe2 Interface

We study the band discontinuity at the GaN/single-layer (SL) WSe₂ heterointerface. The GaN thin layer is epitaxially grown by molecular beam epitaxy on chemically vapor deposited SL-WSe₂/c-sapphire. We confirm that the WSe₂ was formed as an SL from structural and optical analyses using atomic force microscopy, scanning transmission electron microscopy, micro-Raman, absorbance, and microphotoluminescence spectra. The determination of band offset parameters at the GaN/SL-WSe₂ heterojunction is obtained by high-resolution X-ray photoelectron spectroscopy, electron affinities, and the electronic bandgap values of SL-WSe₂ and GaN. The valence band and conduction band offset values are determined to be 2.25 ± 0.15 and 0.80 ± 0.15 eV, respectively, with type II band alignment. The band alignment parameters determined here provide a route toward the integration of group III nitride semiconducting materials with transition metal dichalcogenides (TMDs) for designing and modeling of their heterojunction-based electronic and optoelectronic devices.

ZnO/Cu₂S/ZnO Multilayer Films: Structure Optimization and Its Detail Data for Applications on Photoelectric and Photocatalytic Properties

Monolayer Cu₂S and ZnO, and three kinds of complex films, Cu₂S/ZnO, ZnO/Cu₂S, and ZnO/Cu₂S/ZnO, were deposited on glass substrates by means of radio frequency (RF) magnetron sputtering device. The impact of the thickness of ZnO and Cu₂S on the whole transmittance, conductivity, and photocatalysis was investigated. The optical and electrical properties of the multilayer were studied by optical spectrometry and four point probes. Numerical simulation of the optical transmittance of the multilayer films has been carried out in order to guide the experimental work. The comprehensive performances of the multilayers as transparent conductive coatings were compared using the figure of merit. Compared with monolithic Cu₂S and ZnO films, both the optical transmission property and photocatalytic performance of complex films such as Cu₂S/ZnO and ZnO/Cu₂S/ZnO change significantly.