One-dimensional Heterojunction Research Articles

Subdiffusive and superdiffusive phonon transport induced significant thermal rectification across a one-dimensional Kapitza interface

Anomalous heat conduction in low dimensional materials shows divergent thermal conductivity with increasing characteristic length, which brings many opportunities in thermal management applications and unexplored phonon transport mechanisms. The molecular heterojunctions, consisting of an interface that is formed by different lattices, exhibit unique physical properties especially thermal rectification phenomenon. In this work, we investigate the heat conduction across a one-dimensional heterojunction bridging a carbon nanotube (CNT) and a boron nitride nanotube (BNNT). The molecular dynamic (MD) simulation shows that the rectification ratio is as high as 8.9 in a 300-nm system at/above the room temperature, which is due to the asymmetric interfacial thermal conductance in the opposite directions of heat flow. The interfacial thermal conductance (G) obeys the length dependence of G ∝ Lβ, presenting a divergent and convergent behaviors of anomalous heat conduction that was only reported for homogeneous materials. By analyzing the length-dependent and temperature-dependent phonon density of state, this anomalous interfacial heat conduction can be explained by the Kapitza resistance model and the population of frequency-dependent phonons. Further calculation of mean square displacement of atoms validates the superdiffusion and subdiffusion behavior of the interfacial conductance. This result demonstrates the superb capability of the one-dimensional heterojunction to control phonon transport, which could be used as the basic element of high-efficient and intelligent thermal management devices.

2D/1D nested hollow porous ZnIn2S4/g-C3N4 heterojunction based on morphology modulation for photocatalytic CO2 reduction

The two-dimensional (2D) composite one-dimensional (1D) heterojunction structure is one of the main strategies for constructing composite photocatalysts for photocatalytic reduction of carbon dioxide (CO2). However, morphology modulation for 2D/1D heterojunction is an important way to enhance performance. In this study, a unique 2D/1D ZnIn2S4/g-C3N4 nested hollow porous heterojunction structure was constructed by an in-situ growth method with spatial confinement effect. The synergy between 1D g-C3N4 and 2D ZnIn2S4, achieved through advanced morphology modulation at an optimal ratio of 3:1, results in the formation of the CZ-3 photocatalyst. This unique configuration enables a simultaneous high yield of carbon monoxide (CO) and methane (CH4), achieving 79.96 μmol/g and 17.33 μmol/g respectively, under visible light irradiation without the need for sacrificial agents. The core of this enhanced photocatalytic performance is attributed to the employment of a type-II heterojunction mechanism, which optimizes the transfer of photogenerated carriers. Ultraviolet-visible diffuse reflection spectrum, photoluminescence spectrum and photoelectrochemical analysis reveal the effect of morphology on photocatalytic performance in terms of visible light absorption, photogenerated carrier pairs separation and electronic band structure of heterojunction. By combining the advantages of 2D/1D structural benefits with the efficient charge dynamics of a type-II heterojunction, this work provides new ideas for improving the morphology of composite heterojunction photocatalysts with 2D/1D structure to enhance the photocatalytic CO2 reduction performance.

Space-confined synthesis of SWNT bundles wrapped by MoS2 crystalline layers as flexible sensors and detectors

Single-walled carbon nanotubes (SWNTs) are ideal template for constructing one-dimensional (1D) heterojunctions with intriguing optical and electrical properties, yet it remains challenging to wrap foreign materials around SWNTs due to their very small diameter and large curvature. Here, we present a space-confined method to synthesize SWNT bundles wrapped by MoS2 crystalline layers (SWNT@MoS2) film by embedding a pristine SWNT film within a porous multi-walled nanotube sponge, which acts as a liquid precursor container for hydrothermal reaction. We find that crystalline MoS2 sheets could be formed around the outer surface of SWNT bundles uniformly and continuously, with tailored number of multi-layers (3–17 layers), for bundle sizes ranging from several to hundreds of nanometers. The resulting SWNT@MoS2 network shows much enhanced light response and gas sensitivity, presenting promising applications as flexible photo-detectors and toxic gas sensors. Our scalable synthesis method and SWNT-based coaxial-cables will be useful in future developing innovated 1D van der Waals heterojunctions and flexible nanoelectronic systems.

Macro- and atomic-scale observations of a one-dimensional heterojunction in a nickel and palladium nanowire complex

The creation of low-dimensional heterostructures for intelligent devices is a challenging research topic; however, macro- and atomic-scale connections in one-dimensional (1D) electronic systems have not been achieved yet. Herein, we synthesize a heterostructure comprising a 1D Mott insulator [Ni(chxn)2Br]Br2 (1; chxn = 1R-2R-diaminocyclohexane) and a 1D Peierls or charge-density-wave insulator [Pd(chxn)2Br]Br2 (2) using stepwise electrochemical growth. It can be considered as the first example of electrochemical liquid-phase epitaxy applied to molecular-based heterostructures with a macroscopic scale. Moreover, atomic-resolution scanning tunneling microscopy images reveal a modulation of the electronic state in the heterojunction region with a length of five metal atoms (~ 2.5 nm), that is a direct evidence for the atomic-scale connection of 1 and 2. This is the first time that the heterojunction in the 1D chains has been shown and examined experimentally at macro- and atomic-scale. This study thus serves as proof of concept for heterojunctions in 1D electronic systems.

Time-periodic thermal rectification in heterojunction thermal diodes

Thermal diodes that rectify time-periodic temperature (T) fields have been proposed for applications in waste heat scavenging and thermal control. Although these applications involve transient temperatures, prior analytical solutions for heat conduction in thermal diodes have focused on the steady-state response. We use analytical perturbation methods supported by finite-element method (FEM) calculations to study time-periodic rectification in a one-dimensional heterojunction diode consisting of two materials with thermal conductivity k and heat capacity c that each scale linearly with T. One boundary of the diode experiences a sinusoidal boundary condition with an average temperature T0, an angular frequency ω, and a temperature amplitude ΔT, while the other boundary is maintained at a constant T0. The analytical perturbation solution shows that the nonlinear response has frequency components at direct current (dc) and at the second harmonic of ω for small ΔT/T0. We introduce a rectification asymmetry metric φ as the difference in the forward and reverse diode orientation dc heat fluxes and find that φ can be enhanced at large ω by a factor of two compared to the quasi-steady φ at small ω. We identify the optimal dimensionless parameters to maximize φ and find that φ is independent of the heat capacity temperature coefficient for all ω. We use FEM calculations to validate the perturbation solution and to study the effects of larger ΔT oscillations, interface thermal contact resistances, and heat losses. Our modeling results can be used to design heterojunction thermal diodes for applications and experiments involving time-periodic thermal rectification.

In situ growth of CdS quantum dots on phosphorus-doped carbon nitride hollow tubes as active 0D/1D heterostructures for photocatalytic hydrogen evolution

CdS quantum dots (QDs) were decorated onto phosphorus-doped hexagonal g-C3N4 tube (P-CNT) to form a novel high-preformance photocatalyst (CdS QDs/P-CNT) via an in-situ oil bath approach. The ultra-small CdS QDs with the average diameter of ~9 nm are homogeneously anchored on the both external and internal surface of P-CNT hollow channel (~25 μm), yielding a type of zero-dimensional (0D)/one-dimensional (1D) heterojunction. The CdS QDs/P-CNT-1 exhibits the maximum photocatalytic H2 evolution rate of 1579 μmol h−1 g−1 under visible-light irradiation, which is 31.6, 6.8, 4.7 and 3.1 times higher than P-CNT, CdS, CdS/BCN and CdS/CNT, respectively. The improved photocatalytic activity of CdS QDs/P-CNT is primarily attributed to large surface area, P doping and formed 0D/1D heterojunction, which can broaden the light absorption, narrow the band gap, activate the H2O molecule and promote the spatial charge separation. Moreover, the DFT calculation coupled with experiment (Mott-Schottky curves) illustrates the electron transfer behavior of CdS QDs/P-CNT, showing that the Cd-1 site should be the main active center and P doping is beneficial to increase H2 production. This work provides a new strategy to design of highly active 0D/1D photocatalyst for photocatalytic H2 production.

Hybrid h-BN–Graphene Monolayer with B–C Boundaries on a Lattice-Matched Surface

In-plane heterostructures of hexagonal boron nitride (h-BN) and graphene (Gr) have recently appeared in the focus of material science research owing to their intriguing and tunable electronic properties. However, disclosure of the atomic structure and properties of one-dimensional heterojunctions between Gr and h-BN domains remains a largely unexplored and challenging task. Here, we report an approach to obtain a perfectly oriented and atomically thin hybrid h-BN–Gr heterolayer on the Co(0001) surface. A perfect matching of the lattice parameters ensures an epitaxial growth of both Gr and h-BN on the close-packed Co surface. High crystalline quality of the resulting interface allowed us to uncover the structural and electronic properties of the lateral h-BN/Gr heterojunctions by means of complementary microscopic and spectroscopic techniques. In particular, we established the coexistence of two types of zigzag boundaries made of B–C bonds, while the boundaries with N–C bonds were found to be unfavorable. ...

Strain Mapping of Two-Dimensional Heterostructures with Subpicometer Precision.

Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with subpicometer precision and the ability to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector-the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space data sets contain the full spatially resolved lattice information on the sample. By using this technique on tungsten disulfide (WS2) and tungsten diselenide (WSe2) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multimicron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally epitaxial structures.

Construction of CdS@UIO-66-NH2 core-shell nanorods for enhanced photocatalytic activity with excellent photostability

A novel class of CdS@UIO-66-NH2 core shell heterojunction was fabricated by the facile in-situ solvothermal method. Characterizations show that porous UIO-66-NH2 shell not only allows the visible light to be absorbed on CdS nanorod core, but also provides abundant catalytic active sites as well as an intimate heterojunction interface between UIO-66-NH2 shell and CdS nanorod core. By taking advantage of this property, the core-shell composite presents highly solar-driven photocatalytic performance compared with pristine UIO-66-NH2 and CdS nanorod for the degradation of organic dyes including malachite green (MG) and methyl orange (MO), and displays superior photostability after four recycles. Furthermore, the photoelectrochemical performance of CdS@UIO-66-NH2 can be measured by the UV–vis spectra, Mott-Schottky plots and photocurrent. The remarkably enhanced photocatalytic activity of CdS@UIO-66-NH2 can be ascribed to high surface areas, intimate interaction on molecular scale and the formation of one-dimensional heterojunction with n-n type. What’s more, the core-shell heterostructural CdS@UIO-66-NH2 can facilitate the effective separation and transfer of the photoinduced interfacial electron-hole pairs and protect CdS nanorod core from photocorrosion.

Synergistic doping effects of a ZnO:N/BiVO4:Mo bunched nanorod array photoanode for enhancing charge transfer and carrier density in photoelectrochemical systems

One-dimensional heterojunction nanorods are highly attractive as photoanodes for developing efficient photoelectrochemical (PEC) systems for the effective photogeneration of charge carriers and transport. ZnO/BiVO4 nanorod arrays (NRAs) are excellent candidates if their charge transferring and recombination issues can be improved. In the current work, we have studied the synergistic doping effects of N-doped ZnO/Mo-doped BiVO4 NRAs for enhancing the photoanode activity in PEC devices. The N-doping of ZnO NRs enhances the charge carrier density ∼3-fold over undoped ZnO NRs through increased oxygen vacancies induced by N dopants. The Mo dopants in BiVO4 improve the mobility of photogenerated charge carriers and contribute to reducing charge recombination. The synergistic doping effects of both ZnO and BiVO4 could increase the charge transfer rate constant (kct) of the ZnO:N/BiVO4:Mo heterojunction by ∼40% and decrease the charge transfer resistance ∼1.9-fold compared to those of undoped ZnO/BiVO4, which were confirmed by time resolved photoluminescence (PL) and electrochemical impedance (EIS) analyses. Our optimally fabricated ZnO:N/BiVO4:Mo NRA photoanode could achieve an excellent photocurrent of 3.62 mA cm-2 without the application of any co-catalysts. This work presents a useful strategy for designing efficient heterojunction photoanodes in PEC systems.

Facile Synthesis of Indium Sulfide/Flexible Electrospun Carbon Nanofiber for Enhanced Photocatalytic Efficiency and Its Application.

Heterojunction system has been proved as one of the best architectures for photocatalyst owing to extending specific surface area, expanding spectral response range, and increasing photoinduced charges generation, separation, and transmission, which can provide better light absorption range and higher reaction site. In this paper, Indium Sulfide/Flexible Electrospun Carbon Nanofiber (In2S3/CNF) heterogeneous systems were synthesized by a facile one-pot hydrothermal method. The results from characterizations of SEM, TEM, XRD, Raman, and UV-visible diffuse reflectance spectroscopy displayed that flower-like In2S3 was deposited on the hair-like CNF template, forming a one-dimensional nanofibrous network heterojunction photocatalyst. And the newly prepared In2S3/CNF photocatalysts exhibit greatly enhanced photocatalytic activity compared to pure In2S3. In addition, the formation mechanism of the one-dimensional heterojunction In2S3/CNF photocatalyst is discussed and a promising approach to degrade Rhodamine B (RB) in the photocatalytic process is processed.

Controlling the Interface Areas of Organic/Inorganic Semiconductor Heterojunction Nanowires for High-Performance Diodes.

A new method of in situ electrically induced self-assembly technology combined with electrochemical deposition has been developed for the controllable preparation of organic/inorganic core/shell semiconductor heterojunction nanowire arrays. The size of the interface of the heterojunction nanowire can be tuned by the growing parameter. The heterojunction nanowires of graphdiyne/CuS with core/shell structure showed the strong dependence of rectification ratio and perfect diode performance on the size of the interface. It will be a new way for controlling the structures and properties of one-dimensional heterojunction nanomaterials.

Indium–Tin–Oxide Nanowire Array Based CdSe/CdS/TiO2 One-Dimensional Heterojunction Photoelectrode for Enhanced Solar Hydrogen Production

For photoelectrochemical (PEC) hydrogen production, low charge transport efficiency of a photoelectrode is one of the key factors that largely limit PEC performance enhancement. Here, we report a tin-doped indium oxide (In2O3:Sn, ITO) nanowire array (NWs) based CdSe/CdS/TiO2 multishelled heterojunction photoelectrode. This multishelled one-dimensional (1D) heterojunction photoelectrode shows superior charge transport efficiency due to the negligible carrier recombination in ITO NWs, leading to a greatly improved photocurrent density (∼16.2 mA/cm2 at 1.0 V vs RHE). The ITO NWs with an average thickness of ∼12 μm are first grown on commercial ITO/glass substrate by a vapor–liquid–solid method. Subsequently, the TiO2 and CdSe/CdS shell layers are deposited by an atomic layer deposition (ALD) and a chemical bath deposition method, respectively. The resultant CdSe/CdS/TiO2/ITO NWs photoelectrode, compared to a planar structure with the same configuration, shows improved light absorption and much faster charge transport properties. More importantly, even though the CdSe/CdS/TiO2/ITO NWs photoelectrode has lower CdSe/CdS loading (i.e., due to its lower surface area) than the mesoporous TiO2 nanoparticle based photoelectrode, it shows 2.4 times higher saturation photocurrent density, which is attributed to the superior charge transport and better light absorption by the 1D ITO NWs.

Hierarchical TiO2 photocatalysts with a one-dimensional heterojunction for improved photocatalytic activities

Hierarchical TiO2 photocatalysts with a one-dimensional heterojunction were synthesized via a facile template-free hydrothermal method. The TiO2 photocatalysts were flower-like microspheres with a 3 μm diameter. The base structure of the flower-like microspheres was a uniform nanowire with a 10 nm diameter. Anatase films were evenly coated onto the surface of the rutile TiO2 nanowires to form a one-dimensional core-shell base structure. This kind of one-dimensional heterojunction is conducive to the separation of charge carriers. In addition, the hierarchical TiO2 microspheres possessed a good mesoporous structure with a high specific surface area of 260 m2/g. Thus, the light scattering and utilization efficiency were improved in this structure. The photocatalysts exhibited better performance in both photocatalytic oxidation and reduction reactions. Moreover, the novel TiO2 photocatalysts displayed excellent stability in these reactions. This kind of hierarchical TiO2 structure has never been reported in the literature. The hierarchical structure and one-dimensional heterojunction were vital to the increase in quantum efficiency. Therefore, these hierarchical TiO2 photocatalysts have potential applications in the environmental and energy fields, such as in photocatalytic degradation, hydrogen production, Li-ion batteries, and dye-sensitized solar cells.

Morphology control of one-dimensional heterojunctions for highly efficient photoanodes used for solar water splitting

In a dual bandgap system such as WO3/BiVO4, the morphology of each component should be controlled by understanding its properties, in particular with respect to the charge flow in the system. For WO3/BiVO4 photoanodes, a porous BiVO4 film allows contact of an electrolyte to the bottom layer with enhanced surface area, thereby promoting the oxidation reaction, while one-dimensional (1-D) WO3 nanorods, directly grown on F-doped tin oxide, are advantageous for transporting electrons to the back contact. The morphology of the BiVO4 film covered by 1-D WO3 nanorods varies with the addition of organic additives such as ethylcellulose in the metal precursor solution. The cross-sectional images from scanning electron microscopy show that 1-D WO3 nanorods is coated with the BiVO4 layer, which forms a porous top layer that can effectively absorb visible light and enhance charge transfer resulting in enhanced photocurrents. We report on the highest photocurrent at a potential of 1.23 V versus a reversible hydrogen electrode (RHE) by means of a 1-D WO3/BiVO4/Co-Pi photoanode. The strategies for constructing such kind of heterojunctions are well applicable to other dual bandgap photoanodes.

ZnS/ZnO heterojunction as photoelectrode: Type II band alignment towards enhanced photoelectrochemical performance

Heterojunction structures are attracting lots of attention for enhancing the electron injection across the interface. The ZnS/ZnO one-dimensional heterojunction film was firstly prepared via a chemical sulfidization following hydrothermal reaction. The heterostructure was characterized as ZnS(blende)/ZnO(wurtzite) shell–core nanorods via XRD, SEM and TEM. A type II band alignment structure of ZnS/ZnO composite was synthesized via a temperate condition proved by PLS and XPS. The values for valence band offset (VBO) and conduction band offset (CBO) were calculated to be 0.96 eV and 1.25 eV, respectively. The special electron structure in the heterojunction helped reduce the energy barrier height at the interface and enhance the separation of photo-generated carriers. Thus, the photoelectrochemical performance was highly improved, and a photocurrent density of 380 μA/cm2 at 0.9 V (vs. Ag/AgCl) was obtained.

Formation of Planar Arrays of One-Dimensional p−n Heterojunctions Using Surface-Directed Growth of Nanowires and Nanowalls

We report a surface-directed vapor-liquid-solid process for planar growth of one-dimensional heterojunctions of zinc oxide on single crystal gallium nitride (GaN) that enables their hierarchical assembly to light emitting diodes. An individual heterojunction is about 10 μm in length and 80 nm in width and is formed by planar growth of an n-type ZnO nanowire or nanowall on p-type GaN surface using Au catalyst. Our results show that a ZnO nanocrystal at its nucleation site has six possible growth directions that can be engineered and controlled using an intentional blockade of the nanocrystal growth in certain directions owing to similarities in crystal structures of ZnO and GaN. The ZnO nanowalls are formed when nanowires during their planar growth slowly grow in direction normal to the substrate via a self-catalytic process. The crystal structure of these heterojunctions is examined from two different crystallographic perspectives using high resolution transmission electron microscopy. Results indicate abrupt and epitaxial formation of n-p heterojunctions, which are difficult to achieve in thin film growth of these heterojunctions. The collective light emission of micrometer- to millimeter-size arrays of the heterojunctions is demonstrated via a simple design that is scalable to literally any platform size. This technique allows in situ growth and combinations of II-VI and III-V semiconductors and offers their easier integration to photonic and lab-on-chip platforms with applications in energy generation and light detection.

Electron transport characteristics of one-dimensional heterojunctions with multi-nitrogen-doped capped carbon nanotubes

We present a systematic analysis of electron transport characteristics for one-dimensional heterojunctions with two multi-nitrogen-doped (multi-N-doped) capped carbon nanotubes (CNTs) facing one another at different numbers of nitrogen atoms and conformations. Our results show that the modification of the molecular orbitals by the nitrogen dopants generates conducting channels in the designed heterojunctions inducing multi-switching behavior with sequential negative differential resistance (NDR). The NDR behavior significantly depends on the doping site and conformation of doped nitrogen atoms. Furthermore, we provide a clear interpretation for the NDR behavior by a rigid shift model of the HOMO- and LUMO-filtered energy levels in the left and right electrodes under the applied biases. We believe that our results will give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.

P - N junction with donor and acceptor encapsulated single-walled carbon nanotubes

Ultimate one-dimensional heterojunctions of electron donor and acceptor materials have been realized within the inner hollow space of a single-walled carbon nanotube (SWNT). The heterojunction structures of Cs/I and Cs/C60 inside SWNTs (Cs/I@SWNTs, Cs/C60@SWNTs) yield the air-stable rectifying performance. Clear tunneling currents through the p-n junction barrier could be also detected only for Cs/I@SWNTs, which is explained by the difference of depletion layer structures. Based on a potential calculation, symmetrical and asymmetrical depletion layers were found to be formed in Cs/I@SWNTs and Cs/C60@SWNTs, respectively. Low temperature measurements also supply evidence of asymmetric depletion layer formation in Cs/C60@SWNTs.

Arrays of Ni nanowire/multiwalled carbon nanotube/amorphous carbon nanotube heterojunctions containing Schottky contacts

Ordered arrays of heterojunctions comprising Ni nanowires, multiwalled carbon nanotubes, and amorphous carbon nanotubes (a-CNTs) connected end to end were fabricated. The current-voltage (I-V) characteristics of the heterojunctions embedded in the arrays were measured by a conductive atomic force microscope. It was found that although the electrical signals of Schottky contacts in some heterojunctions were buried by those of the long a-CNT segments, Schottky contacts in the other heterojunctions played a central role and made the corresponding heterojunctions possess rectifying I-V characteristics. The quantitative analysis showed that the thermionic emission theory was applicable to Schottky contacts in one-dimensional heterojunctions.