Binbin Weng

Study of sensitization process on mid-infrared uncooled PbSe photoconductive detectors leads to high detectivity

For nearly a century, oxygen has been widely accepted as the key element that triggers photo-response in polycrystalline PbSe photoconductive detectors. Our photoluminescence and responsivity studies on PbSe samples, however, suggest that oxygen only serves as an effective sensitization improver and it is iodine rather than oxygen that plays the key role in triggering the photo-response. These studies shed light on the sensitization process for detector applications and ways to passivate defects in IV–VI semiconductors. As a result, high peak detectivity of 2.8 × 1010 cm·Hz1/2·W−1 was achieved at room temperature.

Coaxial multi-shelled TiO2 nanotube arrays for dye sensitized solar cells

The performance of one-dimensional (1D) TiO2 nanotube based dye-sensitized solar cells (DSCs) was limited by the insufficient surface area of TiO2 nanotubes. To solve this issue, coaxial multiple-shelled TiO2 nanotube arrays were successfully synthesized on the transparent conductive oxide (TCO) substrates by using improved ZnO nanorod template assisted layer by layer absorption and reaction (LbL-AR) technique. To fabricate tube-in-tube nanostructures, LbL-AR TiO2 coatings were successively deposited on the exterior walls of the ZnO nanowires and the sacrificial sol–gel ZnO spacers, which were removed together by selective etching to form the hollow tubal structures. The performance of dye-sensitized solar cells (DSCs) increases with increasing the shell number of multi-shelled TiO2 nanotube photoanodes, attributed to the increase of the surface area, which was confirmed by N2 adsorption-desorption isotherms and the dye-loading capacities. A maximum efficiency of 6.2% was achieved for a quintuple shelled TiO2 nanotube photoanode with a short-circuit current density (Jsc) = 15 mA cm2, open-circuit voltage (Voc) = 0.73 V and fill factor (FF) = 0.57.

Room temperature mid-infrared surface-emitting photonic crystal laser on silicon

We demonstrate a mid-infrared surface-emitting photonic crystal laser on silicon substrate operating at room temperature. The active region consisting of PbSe/PbSrSe multiple quantum wells was grown by molecular beam epitaxy on Si(111) substrate patterned with a photonic crystal (PC) array. The PC array forms a transverse magnetic polarized photonic bandgap at around 2840 cm−1. Under pulsed optical pumping, room temperature multimode lasing emissions were observed at wavelength ∼3.5 μm with estimated threshold peak pumping intensity of 24 kW/cm2. Angular-dependent measurement indicates the lasing is of a Gaussian-like profile with full width at half maximum of 4.66°.

Branched double-shelled TiO2 nanotube networks on transparent conducting oxide substrates for dye sensitized solar cells

A novel three-dimensional branched double-shelled TiO2 nanotube network (3DNTNs) structure was constructed on transparent conducting oxide substrates (TCO) by using a ZnO-nanorod array template-assisted method. Various morphological features, such as branch length and population density, could be easily tailored by simply modifying the growth time. The performances of dye-sensitized solar cells (DSCs) fabricated with TiO2 3DNTNs are higher than those assembled with 1-dimensional nanotube arrays. The hollow branches filling in the spaces between the double-shelled stem nanotubes increase the amount of dye-loading, resulting in an enhanced light-harvesting ability.

Understanding sensitization behavior of lead selenide photoconductive detectors by charge separation model

We introduce a charge separation model in this work to explain the mechanism of enhanced photoconductivity of polycrystalline lead salt photoconductors. Our results show that this model could clarify the heuristic fabrication processes of such lead salt detectors that were not well understood and often considered mysterious for nearly a century. The improved lifetime and performance of the device, e.g., responsivity, are attributed to the spatial separation of holes and electrons, hence less possibility of carrier recombination. This model shows that in addition to crystal quality the size of crystallites, the depth of outer conversion layer, and doping concentration could all affect detector performance. The simulation results agree well with experimental results and thus offer a very useful tool for further improvement of lead salt detectors. The model was developed with lead salt family of photoconductors in mind, but may well be applicable to a wider class of semiconducting films.

Responsivity enhancement of mid-infrared PbSe detectors using CaF2 nano-structured antireflective coatings

The CaF2 nano-structures grown by thermal vapor deposition are presented. Significant responsivity improvement (>200%) of mid-infrared PbSe detectors incorporating a 200 nm nano-structured CaF2 coating was observed. The detector provides a detectivity of 4.2 × 1010 cm · Hz1/2/W at 3.8 μm, which outperforms all the reported un-cooled PbSe detectors. Structural investigations show that the coating is constructed by tapered-shape nanostructures, which creates a gradient refractive-index profile. Analogy to moth-eye antireflective mechanism, the gradient refractive-index nanostructures play the major roles for this antireflection effect. Some other possible mechanisms that help enhance the device performance are also discussed in the work.

Elimination of threading dislocations in as-grown PbSe film on patterned Si(111) substrate using molecular beam epitaxy

A high-quality as-grown PbSe film with a record low threading dislocation density of 9×105 cm−2 on patterned Si(111) substrate has been obtained using molecular beam epitaxy. The mechanisms leading to the remarkable reduction in threading dislocation density are analyzed. Based on the analysis, further reduction in dislocation density is anticipated. Materials with such low dislocation density should significantly improve the Si-based IV-VI group device performance.

CdS/PbSe heterojunction for high temperature mid-infrared photovoltaic detector applications

n-CdS/p-PbSe heterojunction is investigated. A thin CdS film is deposited by chemical bath deposition on top of epitaxial PbSe film by molecular beam epitaxy on Silicon. Current-voltage measurements demonstrate very good junction characteristics with rectifying ratio of ∼178 and ideality factor of 1.79 at 300 K. Detectors made with such structure exhibit mid-infrared spectral photoresponse at room temperature. The peak responsivity Rλ and specific detectivity D* are 0.055 A/W and 5.482 × 108 cm·Hz1/2/W at λ = 4.7 μm under zero-bias photovoltaic mode. Temperature-dependent photoresponse measurements show abnormal intensity variation below ∼200 K. Possible reasons for this phenomenon are also discussed.

Mid-infrared surface-emitting photonic crystal microcavity light emitter on silicon

We describe an IV-VI semiconductor light emitter consisting of a PbSe/PbSrSe multiple quantum well active region grown by molecular beam epitaxy on a patterned Si(111) substrate with a two dimensional (2D) photonic crystal (PC) array. The 2D PC array was designed to form photonic band gaps around 1960 and 2300 cm−1. Under pulsed optical pumping, light emission was observed with strongly coupled PC defect modes, which correspond well with simulated photonic band gaps. The observed spectral linewidth was around 10 cm−1 and the highest quantum efficiency measured was 12.8%.

Synthesis and characterization of flower-like bundles of zno nanosheets by a surfactant-free hydrothermal process

Flower-like bundles of ZnO nanosheets have been prepared by using preheating hydrothermal process without any surfactants. The flower-like bundles consist of many thin and uniform hexagonal-structured ZnO nanosheets, with a thickness of 50 nm. The selected area electronic diffraction (SAED) and high-resolution transmission electron microscope (HRTEM) images indicate that the ZnO nanosheets are single crystal in nature. The growth mechanism of the flower-like bundles of ZnO nanosheets is discussed based on the morphology evolution with growth times and reaction conditions. It is believed that the formation of flowerlike bundles of ZnO nanosheets is related to the shielding effect of OH- ions and the self-assembly process, which is dominated by a preheating time. Room temperature photoluminescence spectra results show that the annealing atmosphere strongly affects the visible emission band, which is sensitive to intrinsic and surface defects, especially oxygen interstitials, in flower-like bundles of ZnO nanosheets.