scispace - formally typeset
Search or ask a question

Showing papers on "Band offset published in 2016"


Journal ArticleDOI
TL;DR: The Z-scheme heterostructure mechanism can well explain the improved photocatalytic activity of the g-C3N4/TiO2 heterost structure.
Abstract: Constructing a TiO2 based heterostructure is a very effective strategy for enhancing photocatalytic performance. The details of the electronic structure, interfacial interaction, and photogenerated carrier separation are important for explaining the photocatalytic properties of a heterostructure. Herein, the density of states, charge distribution, and the band offset of the monolayer g-C3N4/TiO2 heterojunction are systematically investigated through the hybrid DFT method. Results indicated that the valence band offset and the conduction band offset of the g-C3N4/TiO2 heterostructure were 0.40 and 0.18 eV, respectively. Interfacial interaction made the TiO2 surface with negative charge, whereas the g-C3N4 surface with positive charge, which led to the formation of a built-in electric field at the interface. Under illumination, the built-in electric field accelerates the transfer of photoexcited electrons in the CB of TiO2 into the VB of g-C3N4, thus resulting in the photoexcited electrons and holes naturally accumulating in the CB of g-C3N4 and the VB of TiO2, respectively. The photoexcited electrons and holes gathering in different surface regions efficiently prolonged the lifetime of photogenerated carriers. Meanwhile, electrons in the CB of g-C3N4 and holes in the VB of TiO2 had a stronger redox ability. Therefore, g-C3N4/TiO2 is a direct Z-scheme photocatalyst, and the Z-scheme heterostructure mechanism can well explain the improved photocatalytic activity of the g-C3N4/TiO2 heterostructure.

423 citations


Journal ArticleDOI
Bo Li1, Le Huang1, Mianzeng Zhong1, Yan Li1, Yan Wang1, Jingbo Li1, Zhongming Wei1 
TL;DR: In this paper, a direct vapor phase growth of high-quality vertically stacked heterostructure of SnS2/MoS2 monolayers is reported, where an extremely Type II band alignment exists in this 2D heterostructures, with band offset larger than any other reported.
Abstract: 2D van der Waals heterostructures with different types of band alignment have recently attracted great attention due to their unique optical and electrical properties. Most 2D heterostructures are formed by transfer-stacking two monolayers together, but the interfacial quality and controllable orientation of such artificial structures are inferior to those epitaxial grown heterostructures. Herein, for the first time, a direct vapor phase growth of high-quality vertically stacked heterostructure of SnS2/MoS2 monolayers is reported. An extremely Type II band alignment exists in this 2D heterostructure, with band offset larger than any other reported. Consistent with the large band offset, distinctive optical properties including strong photoluminescence quenching in the heterostructure area are observed in the heterostructure. The SnS2/MoS2 heterostructures also exhibit well-aligned lattice orientation between the two layers, forming a periodic Moire superlattice pattern with high lattice mismatch. Electrical transport and photoresponsive studies demonstrate that the SnS2/MoS2 heterostructures exhibit an obvious photovoltaic effect and possess high on/off ratio (>106), high mobility (27.6 cm2 V−1 s−1) and high photoresponsivity (1.36 A W−1). Efficient synthesis of such vertical heterostructure may open up new realms in 2D functional electronics and optoelectronics.

150 citations


Journal ArticleDOI
03 Jun 2016-ACS Nano
TL;DR: It is demonstrated that the intrinsic band offset at the CH3NH3PbI3/MoS2 interface can be overcome by creating sulfur vacancies in MoS2 using a mild plasma treatment, and highlights the feasibility of applying defect-engineered 2-D TMDCs as charge-extraction layers in perovskite-based optoelectronic devices.
Abstract: The performance of a photovoltaic device is strongly dependent on the light harvesting properties of the absorber layer as well as the charge separation at the donor/acceptor interfaces. Atomically thin two-dimensional transition metal dichalcogenides (2-D TMDCs) exhibit strong light–matter interaction, large optical conductivity, and high electron mobility; thus they can be highly promising materials for next-generation ultrathin solar cells and optoelectronics. However, the short optical absorption path inherent in such atomically thin layers limits practical applications. A heterostructure geometry comprising 2-D TMDCs (e.g., MoS2) and a strongly absorbing material with long electron–hole diffusion lengths such as methylammonium lead halide perovskites (CH3NH3PbI3) may overcome this constraint to some extent, provided the charge transfer at the heterostructure interface is not hampered by their band offsets. Herein, we demonstrate that the intrinsic band offset at the CH3NH3PbI3/MoS2 interface can be o...

121 citations


Journal ArticleDOI
Chengcheng Li1, Tuo Wang1, Zhibin Luo1, Shanshan Liu1, Jinlong Gong1 
01 Jul 2016-Small
TL;DR: This paper describes the design and fabrication of a 3D Fe2 O3 /Fe2 TiO5 heterojunction photoanode with improved charge separation, via a facile hydrothermal method followed by atomic layer deposition and air annealing, which achieves an overall Faradaic efficiency of 95% for O2 generation.
Abstract: Hematite suffers from poor charge transport and separation properties for solar water splitting. This paper describes the design and fabrication of a 3D Fe2 O3 /Fe2 TiO5 heterojunction photoanode with improved charge separation, via a facile hydrothermal method followed by atomic layer deposition and air annealing. A highly crystallized Fe2 TiO5 phase forms with a distinct interface with the underlying Fe2 O3 core, where a 4 nm Fe2 TiO5 overlayer leads to the best photoelectrochemical performance. The favorable band offset between Fe2 O3 and Fe2 TiO5 establishes a type-II heterojunction at the Fe2 O3 /Fe2 TiO5 interface, which drives electron-hole separation effectively. The Fe2 O3 /Fe2 TiO5 composite electrode exhibits a dramatically improved photocurrent of 1.63 mA cm(-2) at 1.23 V versus reversible hydrogen electrode (RHE) under simulated 1 sun illumination (100 mW cm(-2) ), which is 3.5 times that of the bare Fe2 O3 electrode. Decorating the Fe2 O3 /Fe2 TiO5 heterojunction photoanode with earth-abundant FeNiOx cocatalyst further expedites surface reaction kinetics, leading to an onset potential of 0.8 V versus RHE with a photocurrent of 2.7 mA cm(-2) at 1.23 V and 4.6 mA cm(-2) at 1.6 V versus RHE. This sandwich photoanode shows an excellent stability for 5 h and achieves an overall Faradaic efficiency of 95% for O2 generation. This is the best performance ever reported for Fe2 O3 /Fe2 TiO5 photoanodes.

120 citations


Journal ArticleDOI
TL;DR: It is demonstrated how changes in deposition conditions can dramatically influence the functional properties of an interface, even within the same material system.
Abstract: Energy band alignments at heterointerfaces play a crucial role in defining the functionality of semiconductor devices, yet the search for material combinations with suitable band alignments remains a challenge for numerous applications. In this work, we demonstrate how changes in deposition conditions can dramatically influence the functional properties of an interface, even within the same material system. The energy band alignment at the heterointerface between Cu2O and ZnO was studied using photoelectron spectroscopy with stepwise deposition of ZnO onto Cu2O and vice versa. A large variation of energy band alignment depending on the deposition conditions of the substrate and the film is observed, with valence band offsets in the range ΔEVB = 1.45-2.7 eV. The variation of band alignment is accompanied by the occurrence or absence of band bending in either material. It can therefore be ascribed to a pinning of the Fermi level in ZnO and Cu2O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu2O. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as ΔEVB ≈ 1.5 eV, being favorable for solar cell applications.

100 citations


Journal ArticleDOI
TL;DR: The photoresponse characteristics of the a-IGZO transistor are extended to cover most of the visible range by forming a heterojunction phototransistor that harnesses a visible light responding MoS2 film with a small band gap prepared through a large-area synthetic route.
Abstract: We introduce an amorphous indium–gallium–zinc-oxide (a-IGZO) heterostructure phototransistor consisting of solution-based synthetic molybdenum disulfide (few-layered MoS2, with a band gap of ∼1.7 eV) and sputter-deposited a-IGZO (with a band gap of ∼3.0 eV) films as a novel sensing element with a broad spectral responsivity. The MoS2 and a-IGZO films serve as a visible light-absorbing layer and a high mobility channel layer, respectively. Spectroscopic measurements reveal that appropriate band alignment at the heterojunction provides effective transfer of the visible light-induced electrons generated in the few-layered MoS2 film to the underlying a-IGZO channel layer with a high carrier mobility. The photoresponse characteristics of the a-IGZO transistor are extended to cover most of the visible range by forming a heterojunction phototransistor that harnesses a visible light responding MoS2 film with a small band gap prepared through a large-area synthetic route. The MoS2–IGZO heterojunction phototransist...

92 citations


Journal ArticleDOI
TL;DR: In this paper, the band alignment parameters of the GaN/single-layer (SL) MoS2 heterostructure were determined by high-resolution X-ray photoelectron spectroscopy.
Abstract: We report the band alignment parameters of the GaN/single-layer (SL) MoS2 heterostructure where the GaN thin layer is grown by molecular beam epitaxy on CVD deposited SL-MoS2/c-sapphire. We confirm that the MoS2 is an SL by measuring the separation and position of room temperature micro-Raman E12g and A1g modes, absorbance, and micro-photoluminescence bandgap studies. This is in good agreement with HRTEM cross-sectional analysis. The determination of band offset parameters at the GaN/SL-MoS2 heterojunction is carried out by high-resolution X-ray photoelectron spectroscopy accompanying with electronic bandgap values of SL-MoS2 and GaN. The valence band and conduction band offset values are, respectively, measured to be 1.86 ± 0.08 and 0.56 ± 0.1 eV with type II band alignment. The determination of these unprecedented band offset parameters opens up a way to integrate 3D group III nitride materials with 2D transition metal dichalcogenide layers for designing and modeling of their heterojunction based electr...

73 citations


Journal ArticleDOI
TL;DR: In this article, the authors showed a reduction of the conduction band offset from (0.63±0.03) eV to (0 48±0 eV when the Mg content is below 1.6 eV.

64 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1'nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature.
Abstract: The greatest challenge for improving the power conversion efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)/CdS/ZnO thin film solar cells is increasing the open circuit voltage (VOC). Probable leading causes of the VOC deficit in state-of-the-art CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1 nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature. Capacitance-voltage profiling and quantum efficiency measurements reveal that ALD-Al2O3 interface modification reduces the density of acceptor-like states at the heterojunction resulting in reduced interface recombination and wider depletion width. Indications of increased VOC resulting from the modification of the heterojunction interface as a result of ALD-Al2O3 treatment are presented. These results, while ...

61 citations


Journal ArticleDOI
Saurabh Sant1, Andreas Schenk1
TL;DR: In this article, the effect of channel quantization on the line tunneling is considered in the semiclassical simulations using a new model that modifies the band edge in the inversion layer.
Abstract: This paper presents a simulation study of In0.53Ga0.47As/InP heterojunction gate-overlapped-source tunnel FETs (GoS-TFETs) with pocket counter-doping. The effect of channel quantization on the line tunneling is considered in the semiclassical simulations using a new model that modifies the band edge in the inversion layer. The small bandgap of the source material In0.53Ga0.47As results in an improved tunnel rate, while the wide bandgap of the channel/drain material InP reduces ambipolar leakage. The simulations show that, for the case of perfectly aligned p-n-junction and heterojunction, the type-I band alignment and the large band offsets delay suppress lateral (point) tunneling relative to vertical (line) tunneling which improves the subthreshold swing (SS). The counter-doped pocket in the source region advances the onset of line tunneling relative to point tunneling which also assists in mitigating the effects of point tunneling. In this way, both large band offset and counter-doped pocket improve the subthreshold behavior of the TFET. Placing the p-n-junction inside the InP region makes the vertical tunneling even more dominant and, thus, reduces the SS. The suggested modifications might be useful to improve the device performance beyond that of the conventional GoS-TFETs.

53 citations


Journal ArticleDOI
TL;DR: In this paper, the authors evaluated the band offsets at the SiO2/β-Ga2O3 (010) interface by X-ray photoelectron spectroscopy (XPS).
Abstract: We evaluated the band offsets at the SiO2/β-Ga2O3 (010) interface by X-ray photoelectron spectroscopy (XPS). Plasma chemical vapor deposition with a liquid tetraethyl orthosilicate source for SiO2 was used to prepare an SiO2(40 nm)/Ga2O3 sample and an SiO2(3 nm)/Ga2O3 sample for XPS analyses. The bandgap of SiO2 was determined to be 8.7 ± 0.2 eV. A large conduction band offset of 3.1 ± 0.2 eV and a corresponding valence band offset of 1.0 ± 0.2 eV were determined for the SiO2/Ga2O3 interface. These results suggest that an SiO2 gate insulator is favorable for Ga2O3 field effect transistors operating under high temperatures.

Journal ArticleDOI
TL;DR: In this article, X-ray photoelectron spectroscopy has been used to determine the valence band offset at Ga2O3/Si heterojunction interface and the binding energies of Si 2p and Ga 2p3/2 core levels were measured.
Abstract: Ga2O3 thin films were deposited on (111) Si substrate by pulsed laser deposition method. X-ray photoelectron spectroscopy has been used to determine the valence band offset at Ga2O3/Si heterojunction interface. We measured the binding energies of Si 2p and Ga 2p3/2 core levels and the valence band maxima energies. The valence band offset is determined to be 3.5 ± 0.1 eV. As a consequence a type Ι heterojunction with a conduction band offset of 0.2 ± 0.1 eV is found. The determination of the band alignment of Ga2O3/Si heterojunction facilitates the design of optical and electronic devices based on the Ga2O3/Si structure.

Journal ArticleDOI
TL;DR: In this paper, first-principles based density functional theory calculations have been done for the first time on the band offsets of BaTiO 3 /Cu 2 O heterojunction interface and effective mass of electron and hole for bulk BaO 3 and Cu 2 O thin films were synthesized using spray deposition of porous cuprous oxide films onto the surface of spin coated nanostructured thin films.
Abstract: Nanostructured BaTiO 3 /Cu 2 O heterojunction electrodes with varying thickness of Cu 2 O thin films were synthesized using spray deposition of porous cuprous oxide films onto the surface of spin coated nanostructured thin films of BaTiO 3. First-principles based density functional theory calculations have been done for the first time on the band offsets of BaTiO 3 /Cu 2 O heterojunction interface and effective mass of electron and hole for bulk BaTiO 3 and Cu 2 O, exhibited better separation of the photogenerated charge carriers at the BaTiO 3 /Cu 2 O interface. Experimental results on photoelectrochemical activity of BaTiO 3 /Cu 2 O heterojunction in the photoelectrochemical cell for water splitting reaction validate the theoretical results. Maximum photocurrent density value of 1.44 mA/cm 2 at 0.95 V/SCE was observed for BaTiO 3 /Cu 2 O heterojunction photoelectrode with 442 nm thickness. Photo-generated charge carriers apparently transfer more easily in BaTiO 3 /Cu 2 O heterojunction than that in pristine Cu 2 O and BaTiO 3 .

Journal ArticleDOI
TL;DR: The interfacing effects have been studied in MoS2/WS2 quantum-well and superlattice in-plane heterostructures on the basis of first-principles electronic calculations and type-II band alignments are illustrated.
Abstract: In-plane heterostructures of two-dimensional transition-metal dichalcogenides (TMDs) demonstrate the formation of one-dimensional interfaces (or interlines), leading to new exciting properties and device functionalities. In this work, the interfacing effects have been studied in MoS2/WS2 quantum-well and superlattice in-plane heterostructures on the basis of first-principles electronic calculations. In light of the orbital-projected band structures, MoS2/WS2 in-plane heterostructures illustrate type-II band alignments with rather a small band offset for the valence band maximum and a relatively large band offset for the conduction band minimum. Upon increasing the width of TMD constituents, the band gap varies within a small range. In MoS2 and WS2, the surline energy and work function of zigzag edges with S-terminations are obviously higher than those of metal-terminations, and charge transfer from MoS2 to WS2 could be addressed due to the difference in the Fermi level. In-gap levels induced by S vacancies in MoS2/WS2 in-plane heterostructures are discrete and, interestingly, change to consecutive bands due to the built-in electric field.

Journal ArticleDOI
TL;DR: It is shown that hexagonal-diamond NWs are characterized by a more pronounced quantum confinement effect than cubic-d diamond NWs, and the study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface.
Abstract: Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Driven by this promising evidence, we use first-principles methods based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of hexagonal–diamond and cubic–diamond Si NWs as well as their homojunctions. We show that hexagonal–diamond NWs are characterized by a more pronounced quantum confinement effect than cubic–diamond NWs. Furthermore, they absorb more light in the visible region with respect to cubic–diamond ones and, for most of the studied diameters, they are direct band gap materials. The study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface. In particular, at small diameters the band-offset is type-I whereas at experimentally relevant sizes the offset turns up to be of type-II. These findings highlight intriguing possibilities to modulate electron and h...

Journal ArticleDOI
TL;DR: In this article, a type-II InGaN-ZnGeN2 quantum well (QW) was proposed as improved active regions for light-emitting diodes emitting in the blue and green spectral ranges.
Abstract: Type-II InGaN-ZnGeN2 quantum wells (QWs) are studied as improved active regions for light-emitting diodes emitting in the blue (λ ∼ 485 nm) and green (λ ∼ 530 nm) spectral ranges. Both the energy band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II InGaN-ZnGeN2 heterostructure. The strong confinement of holes in the ZnGeN2 layer allows the use of a lower In-content InGaN QW to extend the emission wavelength into the blue and green wavelength regions, as compared to the traditional InGaN QW with uniform In content. In the type-II InGaN-ZnGeN2 QW designs, a thin AlGaN layer was used as a barrier for better carrier confinement. The type-II InGaN-ZnGeN2 QWs lead to a significant enhancement of the electron-hole wave function overlap as compared to those of the conventional QWs. Simulation studies of the proposed type-II QWs promise a significant enhancement of the spontaneous emission rat...

Journal ArticleDOI
TL;DR: In this article, the authors measured the valence band offset at the interface between the BaSi2 and the a-Si to understand the carrier transport properties by the determination of the band offset.
Abstract: The 730 nm-thick undoped BaSi2 films capped with 5 nm-thick amorphous Si (a-Si) intended for solar cell applications were grown on Si(111) by molecular beam epitaxy. The valence band (VB) offset at the interface between the BaSi2 and the a-Si was measured by hard x-ray photoelectron spectroscopy to understand the carrier transport properties by the determination of the band offset at this heterointerface. We performed the depth-analysis by varying the take-off angle of photoelectrons as 15°, 30°, and 90° with respect to the sample surface to obtain the VB spectra of the BaSi2 and the a-Si separately. It was found that the barrier height of the a-Si for holes in the BaSi2 is approximately −0.2 eV, whereas the barrier height for electrons is approximately 0.6 eV. This result means that the holes generated in the BaSi2 layer under solar radiation could be selectively extracted through the a-Si/BaSi2 interface, promoting the carrier separation in the BaSi2 layer. We therefore conclude that the a-Si/BaSi2 inte...

Journal ArticleDOI
TL;DR: In this paper, the authors used a typical in situ-method to prepare diverse graphite-phase carbon nitride (g-C3N4) samples from melamine, thiourea, and a mixture thereof, and found that they exhibited band gaps between 2.3 and 2.8 eV.
Abstract: Band offset is a dominant factor affecting the photocatalytic performance of heterostructure photocatalysts. Therefore, controlling the band gap structure of semiconductors is a key challenge in the development of efficient photocatalysts. We used a typical in situ-method to prepare diverse graphite-phase carbon nitride (g-C3N4) samples from melamine, thiourea, and a mixture thereof, and found that they exhibited band gaps between 2.3–2.8 eV. From UV–Vis spectra and X-ray photoelectron spectroscopy measurements, we determined that the g-C3N4 samples exhibited different band gap values and valence band positions. On this basis, we constructed g-C3N4/m-LaVO4 heterojunctions with different band offsets. UV–Vis spectra and X-ray photoelectron spectroscopy measurements revealed that the valence band offsets (VBOs) of the different heterojunctions were similar, but their conduction band offsets (CBOs) were significantly different. Photocatalytic experiments revealed that the reaction rate was enhanced with an increase in the CBO value. Furthermore, the three-phase g-C3N4/g-C3N4/m-LaVO4 heterojunction composed of m-LaVO4 and mixed g-C3N4 showed the highest photocatalytic activity, which was mainly due to the construction of a multilevel structure. This work investigates the influence of the band offset on heterojunction photoelectrochemical properties and provides a new strategy to improve the photocatalytic activity by constructing multilevel structures.

Journal ArticleDOI
TL;DR: In this article, the authors investigate perovskite planar heterojunction solar cells using 2D physics-based TCAD simulation and show that in order to achieve high efficiency, the mobility of the hole transport layer should exceed 10-4cm2/V s.
Abstract: In this paper, we investigate perovskite planar heterojunction solar cells using 2D physics-based TCAD simulation. The perovskite cell is modeled as an inorganic material with physics-based parameters. A planar structure consisting of $$\hbox {TiO}_{2}$$TiO2 as the electron transport material (ETM), $$\hbox {CH}_{3}\hbox {NH}_{3}\hbox {PbI}_3{}_{-\mathrm{x}}\hbox {Cl}_\mathrm{x}$$CH3NH3PbI3-xClx as the absorber layer, and Spiro-OmeTAD as the hole transport material (HTM) is simulated. The simulated results match published experimental results indicating the accuracy of the physics-based model. Using this model, the effect of the hole mobility and electron affinity/band gap of the hole transport layer (HTM) is investigated. The results show that in order to achieve high efficiency, the mobility of the HTM layer should exceed $$10^{-4}\hbox {cm}^{2}/\hbox {V s}$$10-4cm2/V s. In addition, reducing the band offset to match the valance band of the perovskite results in achieving the highest efficiency. Moreover, the results are discussed in terms of charge transport in the HTM layer and the band alignment at the HTM/perovskite interface.

Journal ArticleDOI
TL;DR: The results suggest that the pronounced intermixing at the In2S3/CIGSe interface leads to a favorable electronic band alignment, necessary for high-efficiency solar cell devices.
Abstract: The interface between a nominal In2S3 buffer and a Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorber was investigated by direct and inverse photoemission to determine the interfacial electronic structure. On the basis of a previously reported heavy intermixing at the interface (S diffuses into the absorber; Cu diffuses into the buffer; and Na diffuses through it), we determine here the band alignment at the interface. The results suggest that the pronounced intermixing at the In2S3/CIGSe interface leads to a favorable electronic band alignment, necessary for high-efficiency solar cell devices.

Journal ArticleDOI
Shanshan Chen1, Xinhua Pan1, Chenxiao Xu1, Jingyun Huang1, Zhizhen Ye1 
TL;DR: In this article, the conduction band offset and valence band offset of the ZnO/Lu2O3 heterojunction were calculated to be 1.77 eV and 0.66 eV, respectively, with a type-I band alignment.

Journal ArticleDOI
TL;DR: Experimental findings and theoretical studies provide unique insights into interface effects across dissimilar gallium chalcogenides and offer new ways to boost optical performance by simple epitaxial coating.
Abstract: Quasi-two-dimensional gallium chalcogenide heterostructures are created by transferring exfoliated few-layer GaSe onto bulk GaTe sheets. Luminescence spectroscopy measurements reveal that the light emission from underlying GaTe layers drastically increases on heterojunction regions where GaSe layers make contact with the GaTe. Density functional theory (DFT) and band offset calculations show that conduction band minimum (CBM) (valance band maximum (VBM)) values of GaSe are higher (lower) in energy compared to GaTe, forming type-I band alignment at the interface. Consequently, GaSe layers provide photo-excited electrons and holes to GaTe sheets through relatively large built-in potential at the interface, increasing overall exciton population and light emission from GaTe. Observed results are not specific to the GaSe/GaTe system but observed on GaS/GaSe heterolayers with type-I band alignment. Observed experimental findings and theoretical studies provide unique insights into interface effects across dissimilar gallium chalcogenides and offer new ways to boost optical performance by simple epitaxial coating.

Journal ArticleDOI
TL;DR: In this article, the authors used first-principles calculations to investigate the band structure evolution of transition metal dichalcogenides under a perpendicular electric field and showed that external electric fields can be used to tune the physics of intralayer and interlayer excitons.
Abstract: We use first-principles calculations to investigate the band structure evolution of $\mathrm{W}{X}_{2}$/$\mathrm{Mo}{X}_{2}$ ($X$ = S, Se) heterobilayers under a perpendicular electric field. We characterize the extent to which the type II band alignment in these compounds can be tuned or inverted electrostatically. Our results demonstrate two effects of the stacking configuration. First, different stackings produce different net dipole moments, resulting in band offset variations that are larger than 0.1 eV. Second, based on symmetry constraints that depend on stacking, a perpendicular electric field may hybridize $\mathrm{W}{X}_{2}$ and $\mathrm{Mo}{X}_{2}$ bands that cross at the Brillouin zone corner $K$. Our results suggest that external electric fields can be used to tune the physics of intralayer and interlayer excitons in heterobilayers of transition metal dichalcogenides.

Journal ArticleDOI
TL;DR: In this article, a simple and low-cost solution-processing approach to synthesize ZnTe nanocrystals by using dendrite-like ZnO nanorods as templates via an in situ method for application in solar cells was reported.
Abstract: ZnTe, a non-toxic low band gap semiconductor, has a direct band gap of 2.26 eV, and can be a promising candidate for non-toxic semiconductor sensitized solar cells (SSSCs). Herein, we report a simple and low-cost solution-processing approach to synthesize ZnTe nanocrystals by using dendrite-like ZnO nanorods as templates via an in situ method for application in solar cells. Structural and morphological analyses and systematic optical property investigations evidenced the successful synthesis of ZnTe nanocrystals and ZnO/ZnTe heterostructures. The measured band alignment of the heterostructures directly points to the strong effect of strain and the possibility to engineer the band offset at the ZnO/ZnTe interface. As ZnO and ZnTe exhibit a type-II energy level alignment, both significant absorption and efficient charge transfer are enabled between the two. Finally, solar cells based on the ZnO/ZnTe heterostructure were fabricated and a short-circuit photocurrent density of over 5 mA cm−2 was achieved, benefiting from the preeminent absorption, high charge separation and transfer efficiency. A ZnS passivation layer dramatically improved the performance of the solar cells reaching a short-circuit photocurrent density of over 10 mA cm−2, along with an increase in the power conversion efficiency (PCE) from 0.46% to 1.7%. Potential pathways towards further increasing this figure are discussed.

Journal ArticleDOI
TL;DR: Pseudopotential atomistic calculation on a model type-II semiconductor heterostructure is used to predict the optimal conditions for controlling multiexciton generation efficiencies at twice the band gap energy and to enable a route for breaking the Shockley–Queisser limit.
Abstract: Multiexciton generation, by which more than a single electron-hole pair is generated on optical excitation, is a promising paradigm for pushing the efficiency of solar cells beyond the Shockley-Queisser limit of 31%. Utilizing this paradigm, however, requires the onset energy of multiexciton generation to be close to twice the band gap energy and the efficiency to increase rapidly above this onset. This challenge remains unattainable even using confined nanocrystals, nanorods or nanowires. Here, we show how both goals can be achieved in a nanorod heterostructure with type-II band offsets. Using pseudopotential atomistic calculation on a model type-II semiconductor heterostructure we predict the optimal conditions for controlling multiexciton generation efficiencies at twice the band gap energy. For a finite band offset, this requires a sharp interface along with a reduction of the exciton cooling and may enable a route for breaking the Shockley-Queisser limit.

Journal ArticleDOI
TL;DR: In this article, the valence band offset value at epitaxial NiO/Al2O3 heterojunction was determined from photoelectron spectroscopy experiments, and the value of conduction band offset was also evaluated from the measured values of band gaps of NiO and Al 2O3 layers.
Abstract: The valence band offset value of 2.3 ± 0.2 eV at epitaxial NiO/Al2O3 heterojunction is determined from photoelectron spectroscopy experiments. Pulsed laser deposited thin film of NiO on Al2O3 substrate is epitaxially grown along [111] direction with two domain structures, which are in-plane rotated by 60° with respect to each other. Observation of Pendellosung oscillations around Bragg peak confirms high interfacial and crystalline quality of NiO layer deposited on Al2O3 substrate. Surface related feature in Ni 2p3/2 core level spectra along with oxygen K-edge soft X-ray absorption spectroscopy results indicates that the initial growth of NiO on Al2O3 substrate is in the form of islands, which merge to form NiO layer for the larger coverage. The value of conduction band offset is also evaluated from the measured values of band gaps of NiO and Al2O3 layers. A type-I band alignment at NiO and Al2O3 heterojunction is also obtained. The determined values of band offsets can be useful in heterojunction based l...

Journal ArticleDOI
TL;DR: In this paper, the authors measured the interfacial energy offset between CH3NH3PbI3 valence band maximum and the highest occupied molecular orbital for five different archetypal organic semiconductors.
Abstract: Organo-metal halide perovskites have recently emerged as a highly promising class of semiconductors for optoelectronic device applications. Integrating these hybrid materials within organic molecular thin film devices is key to fabricate functional devices. By evaporating ultra-thin films of organic semiconductor on thermally evaporated CH3NH3PbI3 and using in-situ ultraviolet photoemission spectroscopy, we directly measure the interfacial energy offset between CH3NH3PbI3 valence band maximum and the highest occupied molecular orbital for 5 different archetypal organic semiconductors. It is found that the energy offsets scale linearly as a function of the ionization energies of the organic semiconductors. The experimental data are in excellent agreement with a theoretical model for ideal semiconductor heterojunctions.

Journal ArticleDOI
TL;DR: In this article, the authors developed a controlled synthesis of α-In2S3/In2O3 nanowire heterostructures by a hydrothermally assisted sulfurization process.
Abstract: Separation of photo-induced charges is crucial in controlling the performance of photocatalysts, photochemical cells, and photovoltaic devices. We developed a controlled synthesis of α-In2S3/In2O3 nanowire heterostructures by a hydrothermally assisted sulfurization process. Structural characterization reveals that two distinct nanowire heterostructures: α-In2S3 nanoparticles decorated In2O3 nanowires and α-In2S3/In2O3 core–shell nanowires were obtained by controlling the pH condition during the sulfurization process. Optical characterization results show α-In2S3/In2O3 nanowire heterostructures exhibit a significantly decreased visible light emission and enhanced visible light absorption compared with the pure In2O3 nanowires, revealing that an efficient photo-induced charge separation efficiency exists. The band offsets of the α-In2S3/In2O3 nanowire heterostructures were determined by X-ray photoemission spectroscopy and a type-II band alignment at the interface is confirmed. Time-resolved photoluminescence results reveal that the α-In2S3 nanoparticles/In2O3 nanowires exhibit significant photo-induced carrier lifetime improvement compare with the α-In2S3/In2O3 core–shell nanowire, due to a shorter charge carrier transport path, which ensures rapid charge separation at the interface. Because of the staggered band offset which promoted effective charge separation, the α-In2S3/In2O3 nanowire heterostructures exhibited enhanced photocatalytic activities under visible light illumination, demonstrating their promising potentials in relevant photo-conversion applications.

Journal ArticleDOI
TL;DR: In this article, the effect of band offset E on thermoelectric properties is investigated based on the three-band model, i.e., one light conduction band, one heavy band, and one valence band.

Journal ArticleDOI
TL;DR: Hematite (α-Fe2 O3) is engineered to improve photoexcited electron-hole pair separation by synthesizing Fe2O3-Cr2O 3 superlattices (SLs) with precise atomic control to generate a built-in potential as large as 0.8 eV in Fe2 O 3-Cr 2O3 SLs.
Abstract: We demonstrate that the different surface terminations exhibited by α-Fe2O3 (hematite) and α-Cr2O3 (eskolaite) in superlattices (SL) of these materials, synthesized with exquisite control by molecular beam epitaxy, determine the heterojunction interface structure and result in controllable, non-commutative band offset values. Precise atomic control of the interface structure allowed us to vary the valence band offset from 0.35 eV to 0.79 eV. This controllable band alignment can be harnessed to generate a built-in potential in Fe2O3-Cr2O3 SLs. For instance, in a 2.5-period SL, a built-in potential of 0.8 eV was realized as measured by x-ray photoelectron spectroscopy of Ti dopants as probe species. The high quality of the SL structure was confirmed by atom probe tomography and scanning transmission electron microscopy. Enhanced photocurrents were measured for a thick Fe2O3 epitaxial film capped with an (Fe2O3)3-(Cr2O3)3 SL; this enhancement was attributed to efficient electron-hole separation in the SL as a result of the band alignment. The Fe-O-Cr bonds at the SL interfaces also red-shifted the onset of photoconductivity to ~1.6 eV. Exploiting the band alignment and photoabsorption properties of Fe2O3-Cr2O3 SLs has the potential to increase the efficiency of hematite-based photoelectrochemical water splitting.