Heterojunction Arrays Research Articles

Silicon nanowire/TiO2 heterojunction arrays for effective photoelectrocatalysis under simulated solar light irradiation

The silicon nanowire (SiNW)/TiO2 heterojunction arrays have been prepared by chemical vapor depositing a TiO2 layer on SiNW arrays which are obtained by chemical etching of Si wafers. The types of SiNW/TiO2 heterojunctions can be modulated simply by using p-type or n-type Si wafers. Compared with samples of depositing TiO2 on p-type or n-type Si wafers (p-Si/TiO2 or p-Si/TiO2), both n-SiNW/TiO2 and p-SiNW/TiO2 heterojunctions can improve the ultraviolet photoresponse under a bias potential, and in particular, a remarkable visible photoresponse of n-SiNW/TiO2 heterojunction is observed when the bias potential is higher than 1.7V (vs. SCE). Cyclic voltammetry (CV) curves illuminate that n-SiNW/TiO2 heterojunctions have steady visible photoresponse in aqueous solution. According to the surface photovoltage (SPV) measurements, it is found that n-SiNW/TiO2 heterojunctions possess window effect, namely, n-SiNW/TiO2 heterojunctions can utilize the superposition photoresponse in the wavelength ranges from 300 to 400nm (contributed by TiO2) and from 400 to 800nm (contributed by n-SiNW). The mechanism of this phenomenon is explained based on the energy band model. The photoelectrocatalytic activity of the n-SiNW/TiO2 heterojunction arrays is evaluated using phenol as a test substance under Xe lamp irradiation. The kinetic constant and total organic carbon (TOC) removal are 1.7 times and 2 times as great as those of n-Si/TiO2, respectively.

Recent Advances in the Use of TiO2 Nanotube and Nanowire Arrays for Oxidative Photoelectrochemistry

In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectrochemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO2−polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results related to the growth of TiO2 nanotube arrays on nonplanar substrates designed in such a way that reflected light normally lost to free space is instead directed to a different point on the device, in turn improving overall photoconversion efficiency. Insofar as the crystallinity of the nanotubes is concerned, the use of a high-temperature oxygen or air-ambient anneal to crystallize the nanotube arrays is disadvantageous, since it results in a thick barrier layer where recombination losses occur and also because it precludes compatibility with polymeric substrates. In this regard, we discovered a two-step fabrication process for synthesis of crystallized nanotube arrays at low-temperatures. The photoelectrochemical applications of TiO2 are limited by its large electronic band gap. We briefly review the cationic and anionic doping approaches popularly used to modify the TiO2 band gap. We consider the use of ternary oxide systems containing titania as both a structural support and corrosion-inhibitor, in particular fabrication and performance of n-type Ti−Fe−O nanotubes and p-type copper-rich Cu−Ti−O nanotubes, with a note on our recent synthesis of iron oxide nanotube arrays by anodic oxidation of iron. Fabrication and photoelectrochemical properties of CdS−TiO2 and CdTe−TiO2 nanotube array heterojunction photoelectrodes are discussed. The article concludes by examining low temperature synthesis, and resulting properties, of single crystal vertically oriented TiO2 nanowire arrays on transparent conductive glass substrates; preliminary investigation of these nanowire array photoelectrodes for water photolysis reveals them to have low series resistance and provide excellent separation of photogenerated charges.

TiO 2–carbon nanotube heterojunction arrays with a controllable thickness of TiO 2 layer and their first application in photocatalysis

TiO 2–carbon nanotube (CNT) heterojunction arrays on Ti substrate were fabricated by a two-step thermal chemical vapor deposition (CVD) method. CNT arrays were first grown on Ti substrate vertically, and then a TiO 2 layer, whose thickness could be controlled by varying the deposition time, was deposited on CNTs. Measured by electrochemical impedance spectroscopy (EIS), the thickness of the TiO 2 layer could affect the photoresponse ability significantly. About 100 nm thickness of the TiO 2 layer proved to be best for efficient charge separation among the tested samples. The optimized TiO 2–CNT heterojunction arrays displayed apparently higher photoresponse capability than that of TiO 2 nanotube arrays which was confirmed by surface photovoltage (SPV) technique based on Kelvin probe and EIS. In the photocatalytic experiments, the kinetic constants of phenol degradation with TiO 2–CNT heterojunctions and TiO 2 nanotubes were 0.75 h −1 ( R 2 = 0.983) and 0.39 h −1 ( R 2 = 0.995), respectively. At the same time, 53.7% of total organic carbon (TOC) was removed with TiO 2–CNT heterojunctions, while the removal of TOC was only 16.7% with TiO 2 nanotubes. These results demonstrate the super capability of the TiO 2–CNT heterojunction arrays in photocatalysis with comparison to TiO 2-only nanomaterial.

Fabrication of silicon carbide nanowires/carbon nanotubes heterojunction arrays byhigh-flux Si ion implantation

An array of silicon carbide nanowire (SiCNW)–carbon nanotube (CNT) heterojunctionswas fabricated by high-flux Si ion implantation into a multi-walled carbon nanotube(MWCNT) array with a metal vapor vacuum arc (MEVVA) ion source. Under Siirradiation, the top part of a CNT array was gradually transformed into an amorphousnanowire array with increasing Si dose while the bottom part still remained aCNT structure. X-ray photoelectron spectroscopy (XPS) analysis shows that theSiC compound was produced in the nanowire part even at the lower Si dose of5 × 1016 ions cm−2, and the SiC amount increased with increasing the Si dose. Therefore, the fabrication of aSiCNW–CNT heterojunction array with the MEVVA technique has been successfullydemonstrated. The corresponding formation mechanism of SiCNWs was proposed.

Electrochemical deposition characteristics of p-CuSCN on n-ZnO rod arrays films

p-CuSCN/n-ZnO rod array heterojunctions were electrodeposited with a weak basic (pH ∼9) aqueous electrolyte solution. I– V characteristics showed the heterostructure had clear rectification, indicating good electrical contacts between ZnO rod arrays and the embedded CuSCN. The energy band model for the electrodeposition of CuSCN on ZnO rod arrays was proposed based on linear sweep voltammetric (LSV) measurements, which indicated that the electrodeposition process was the prior growth of CuSCN on bare ZnO rods according to a conduction process, followed by compact filling in the gaps of the arrays based on the thermal activation mechanism of surface states. The diode properties of the heterojunctions revealed that although deposition was dominated by thermal activation mechanism of surface states, the electrodeposition should be performed at a lower temperature in order to reach fine filling of the gaps of ZnO rod arrays.

Field emission from ordered carbon nanotube-ZnO heterojunction arrays

Low operating electric field and stable emission current have been achieved on ordered carbon nanotube (CNT)-ZnO heterojunction arrays. Pyramid-like micropatterns were firstly prepared via capillarity-driven self-assembly from aligned CNT arrays, ZnO nanowires were subsequently grown via vapor-phase transport (VPT) method. Based on the effect of geometrical structure, low turn-on and threshold fields, and stable emission current were obtained. The high aspect ratio of ZnO nanowires leads to the high field enhancement factor. The optimal density of ZnO nanowires and the excellent interconnection between ZnO and CNT are beneficial to avoid the screening effect and improve the current stability.

Fabrication and electrical, photosensitive properties of p-poly(9,9-diethylfluorene)/n-silicon nanowire heterojunction

P -poly(9, 9-diethylfluorene)/n-silicon nanowire (SiNW) heterojunctions were prepared by spin coating p-type conductive organic polymer on as-prepared n-SiNWs, which were fabricated by electroless metal deposition method. This hybrid nanowire p-n heterojunction shows good rectifying behavior with turn-on voltage of about 1V at room temperature. Temperature-dependent current-voltage properties in the range of 253–413K were investigated and they follow the standard p-n heterojunction electron transport mechanism. The hybrid nanowire heterojunction shows good photovoltaic properties and good sensitivity toward visible light, and a responsivity of 3.23mA∕W under a reverse bias of 10V was obtained. These results present potential applications in the future nanoelectronic and photonic devices based on organic and inorganic p-n heterojunction arrays.

Photocurrent response of the carbon nanotube–silicon heterojunction array

A highly ordered array of parallel, identical carbon nanotubes is grown non-lithographically in a bottom-up fabrication approach to form a heterojunction with a silicon substrate. Evidence of a space-charge separated region at the nanotube–silicon interface is present in the form of diode rectification and a closed-circuit zero-bias photocurrent in response to infrared light. Because carbon nanotubes are narrow bandgap semiconductors, their heterojunction with silicon was analysed spectrally via Fourier transform infrared photocurrent spectroscopy with the aim of investigating the suitability of this structure for infrared (IR) detector applications. IR photoresponse shows signs of temperature-dependent activation that is complex but consistent with estimates of the heterojunction barrier height. Considering the many interesting benefits and properties of carbon nanotubes, these results despite their earliness suggest that nanotube–silicon heterojunction systems could form the foundation for a new kind of infrared detection device.

Vertically Aligned Zn2SiO4 Nanotube/ZnO Nanowire Heterojunction Arrays

Up the junction: An approach for fabricating Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays (see picture) is introduced by using ZnO-nanowire arrays as a template. The technique, which is based on a plasma-enhanced chemical vapor deposition process, reveals a novel, low-cost, and efficient method for aligning nanotube/nanowire heterojunctions, which could be useful for fabricating novel nanodevices.

Electrochemical synthesis of metal and semimetal nanotube–nanowire heterojunctions and their electronic transport properties

Metal and semimetal nanotube-nanowire heterojunction arrays have been achieved by sequential electrochemical-deposition inside the nanochannels of anodic aluminium oxide template with a layer of Au thin enough to leave the pores open.

Water‐Assisted Growth of Aligned Carbon Nanotube–ZnO Heterojunction Arrays

Using water-assisted chemical vapor deposition, aligned CNT–ZnO (CNT: carbon nanotube) heterojunctions are formed on a Zn substrate. The aligned CNT heterojunctions with intimately connected ZnO particles/rods show interesting optoelectronic properties of practical significance. This cost-effective and efficient approach can be scaled up for the production of large-area heterojunction arrays (see figure).

Growth of Uniformly Aligned ZnO Nanowire Heterojunction Arrays on GaN, AlN, and Al0.5Ga0.5N Substrates

Vertically aligned single-crystal ZnO nanorods have been successfully fabricated on semiconducting GaN, Al0.5Ga0.5N, and AlN substrates through a vapor-liquid-solid process. Near-perfect alignment was observed for all substrates without lateral growth. Room-temperature photoluminescence measurements revealed a strong luminescence peak at approximately 378 nm. This work demonstrates the possibility of growing heterojunction arrays of ZnO nanorods on AlxGa1-xN, which has a tunable band gap from 3.44 to 6.20 eV by changing the Al composition from 0 to 1, and opens a new channel for building vertically aligned heterojunction device arrays with tunable optical properties and the realization of a new class of nanoheterojunction devices.

Controlled direct growth of vertical and highly-ordered ’carbon nanotube - silicon’ heterojunction array

AbstractCarbon nanotubes (CNT) have been studied intensively for its unique electrical and mechanic properties and numerous applications. However, electronical integration of carbon nanotubes with silicon to form an electronically functional entity or device, though highly desirable, has remained a great challenge. Whereas vertical aligned bundles of nanotubes have been grown directly on silicon, the integration is mechanical, rather than electronical, in nature. In the present work, we report on Controlled growth of vertical and highly-ordered array of ’carbon nanotube silicon’ (CNS) heterojunction diodes of uniform diameter, length, and alignment. The as grown CNS heterojunction structure is not only electronically functional but also strongly rectifying with an on off ratio as high as five orders of magnitude in current voltage characteristics measured at room temperature. The controlled direct growth of the CNS hetero junction array of such high uniformity is enabled by the formation of a highly ordered nanopore array alumina matrix directly on the silicon, following a precisely controlled anodization of an aluminum thin film evaporated on silicon. Iterations of development and optimization of the anodization process conditions and real time monitoring and control of the anodization process were required for the formation and adhesion of the alumina growth matrix directly on silicon. Under carefully controlled and optimized conditions developed in this work, we are able to obtain an abrupt heterointerface between a terminal end of the carbon nanotube and the silicon substrate, without an intermediate barrier layer, as required for an electronically functional CNS hetero junction. The growth process is applicable to silicon doped in either type, enabling access to another critical parameter of the electronic function.

Vertically aligned carbon nanotube heterojunctions

The bottom-up fabrication and electrical properties of end-to-end contacted multiwalled carbon nanotube (MWCNT) heterojunctions are reported. The vertically aligned MWCNT heterojunction arrays are formed via successive plasma-enhanced chemical vapor deposition processing to achieve the layered junction architecture. Electron microscopy and current-sensing atomic force microscopy are used to reveal the physical nature of the junctions. Symmetric, nonlinear I–V curves of the as-fabricated junctions indicate that a tunnel barrier is formed between the end-to-end contacted MWCNTs. Repeated high bias I–V scans of many devices connected in parallel fuses the heterojunctions, as manifested by a shift to linear I–V characteristics.

Nonaggregated Zinc Phthalocyanine in Mesoporous Nanocrystalline TiO2 Thin Films

Summary: A simple sonochemical approach has been successfully developed to deposit poorly dissolved phthalocyanine dyes onto the surface of a highly-ordered mesoporous nanostructured TiO2 thin film. The aggregation-free phthalocyanines are effectively encapsulated and stabilized in the nanopore arrays of the zeolite-like mesoporous nanocrystalline TiO2 thin film. Highly dispersed dye∥TiO2 heterojunction arrays with a large contact area can be obtained by using this interesting approach. The incorporation of phthalocyanine dyes into the surface of a highly-ordered mesoporous nanostructured TiO2 thin film, a potential photocatalyst.

Electronic transport in a controllably grown carbon nanotube-silicon heterojunction array.

A uniform array of a new type of heterojunction formed between carbon nanotubes and silicon is studied. The heterojunction array was controllably grown with parallel and uniform nanotubes vertically aligned to the silicon substrate using a self-organized nanopore array template. The pronounced rectifying characteristics of the heterojunction were measured with an on/off ratio as high as 10(5) at 4 V. The analysis shows a large and type-I band offset at the heterojunction. The charge transport in the nanotubes is found to be strongly coupled to and limited by the dielectric charging and polarization in the hosting alumina matrix surrounding the nanotubes.

Key performance-limiting defects in P-on-N HgCdTe LPE heterojunction infrared photodiodes

The operability of long-wavelength p-on-n double layer heterojunction arrays for 40K low-background applications has long been limited by the wide variation in pixel-to-pixel zero bias resistance (Ro) values. Diodes on test structures showing lower performance, with Ro values below 7 × 106 ohm at 40K, usually contained gross metallurgical defects such as dislocation clusters and loops, pin holes, striations, Te inclusions, and heavy terracing. However, diodes with Ro values between 7 × 106 and 1× 109 ohm at 40K contained no visible defects. To study the “invisible” performance-limiting defects (i.e. defects that cannot be revealed by etching), a good correlation between the dynamic resistance at 50 mV reverse bias (R50) value at 77K and the Ro value at 40K was first established, and then used as a tool. The correlation allowed measurements of a large number of devices at 77K, rather than relying exclusively on time-consuming measurements at 40K. Interesting results regarding Ro values at 40K, such as insensitivity to low-density dislocations, mild degradation from Hg vacancies, severe degradation from Hg interstitials, and correlation with junction positioning, were obtained from specially designed experiments.

Thermal performance and limitations of silicon–substrate packaged GaAs laser arrays

Thermal resistance and crosstalk have been investigated for a source package consisting of a monolithic, multilaser heterojunction array mounted on a single crystalline silicon substrate, which is in turn laminated to a copper heatsink. Models for 2-D and 3-D heat spreading are used to calculate the heat flow distribution and to obtain upper and lower bounds for both resistance of single devices and crosstalk in arrays. Results for experimental five-laser arrays are shown to fall within these limits. Active cooling is required to maintain junctions at safe operating temperatures prerequisite to stable, long-lived operation.