Xiaoliang Mo

Structures and electrical properties of Ag-tetracyanoquinodimethane organometallic nanowires

Ag-tetracyanoquinodimethane (Ag-TCNQ) nanostructures are synthesized using both solution reaction in acetonitrile and a novel vacuum-saturated vapor reaction method. Experiments show that the latter synthesis method produces Ag-TCNQ nanowires with better uniformity and higher aspect ratio. These nanowires, having diameters around 100 nm and lengths about 5 /spl mu/m, could serve as potential building blocks of nanoscale electronics. Nanodevices based on these nanowires are fabricated using the electron-beam lithography technique. Electrical transport study shows reproducible I--V hysteresis with a change in resistance of four orders of magnitude, demonstrating electrical memory effect. This electrical bistability makes Ag-TCNQ nanowires a promising candidate for future applications in ultrahigh-density information storage.

Preparation and electrical/optical bistable property of potassium tetracyanoquinodimethane thin films

Potassium tetracyanoquinodimethane (K(TCNQ)) thin films were prepared using physical vapor deposition combined with solid state chemical replacement reaction. Reversible electrical bistable behavior at or even above room temperature was observed; and, the optical bistable property of K(TCNQ) film was observed.

Progress and Design Concerns of Nanostructured Solar Energy Harvesting Devices

Integrating devices with nanostructures is considered a promising strategy to improve the performance of solar energy harvesting devices such as photovoltaic (PV) devices and photo-electrochemical (PEC) solar water splitting devices. Extensive efforts have been exerted to improve the power conversion efficiencies (PCE) of such devices by utilizing novel nanostructures to revolutionize device structural designs. The thicknesses of light absorber and material consumption can be substantially reduced because of light trapping with nanostructures. Meanwhile, the utilization of nanostructures can also result in more effective carrier collection by shortening the photogenerated carrier collection path length. Nevertheless, performance optimization of nanostructured solar energy harvesting devices requires a rational design of various aspects of the nanostructures, such as their shape, aspect ratio, periodicity, etc. Without this, the utilization of nanostructures can lead to compromised device performance as the incorporation of these structures can result in defects and additional carrier recombination. The design guidelines of solar energy harvesting devices are summarized, including thin film non-uniformity on nanostructures, surface recombination, parasitic absorption, and the importance of uniform distribution of photo-generated carriers. A systematic view of the design concerns will assist better understanding of device physics and benefit the fabrication of high performance devices in the future.

High-quality organohalide lead perovskite films fabricated by layer-by-layer alternating vacuum deposition for high efficiency photovoltaics

Herein, we present, for the first time, a new procedure to fabricate stoichiometric, uniform, and highly crystalline lead perovskite thin films based on a multi-step vapor deposition technique. In this process, the perovskite film is prepared by layer-by-layer alternating (LBLA) vacuum deposition of PbI2 and methylammounium iodide (MAI). In comparison to the common two-step deposition (TSD) approach, the novel LBLA evaporation process yields thin films with much improved uniformity in terms of composition, morphology, and surface quality. X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry show that the I/Pb ratio of the LBLA film is 2.92 ± 0.03, which indicates a more precise stoichiometric composition of organometallics as compared with TSD with an I/Pb ratio of 2.76 ± 0.06. X-ray diffraction and time-resolved photoluminescence measurements also demonstrate higher crystallinity and a longer lifetime for the LBLA films, as compared with the films fabricated by the conventional TSD method. Atomic force microscopy indicates 36% improvement in the surface smoothness as well. The better film quality leads to ∼20% improved power conversion efficiency in bulk heterojunction solar devices, primarily owing to a higher fill factor and current density.

Solar Energy: Progress and Design Concerns of Nanostructured Solar Energy Harvesting Devices (Small 19/2016).

Nanoengineered materials and structures can harvest light efficiently for photovoltaic applications. Device structure design optimization and material property improvement are equally important for high performance. On page 2536, X. Mo, Z. Fan, and co-workers summarize the design guidelines of solar energy harvesting devices to assist with a better understanding of device physics.

Significantly improved black phase stability of FAPbI3 nanowires via spatially confined vapor phase growth in nanoporous templates

The formamidinium lead iodide (FAPbI3) perovskite has attracted immense research interest as it has much improved stability than methylammonium lead iodide (MAPbI3) while still maintaining excellent optoelectronic properties. Compared to MAPbI3, FAPbI3 has shown an elevated decomposition temperature and a slower decomposition process and therefore it is considered as a more promising candidate for future high-efficiency and reliable optoelectronic devices. However, these excellent optoelectronic properties only exist in the alpha phase and this phase will spontaneously transform into an undesired delta phase with much poorer optoelectronic properties regardless of the environment. This is the main challenge for the application of the FAPbI3 perovskite. Herein, we report a novel strategy to stabilize the cubic black phase of FAPbI3 by using nanoengineering templates. Without further treatment, the black phase can be held over 7 months under ambient conditions and 8 days in an extreme environment with a Relative Humidity (RH) of 97%. A systematic study further reveals that this great improvement can be attributed to the spatial confinement in anodized alumina membrane (AAM) nanochannels, which prohibits the unwanted α-to-δ phase transition by restricting the expansion of NWs in the ab plane, and the excellent passivation against water molecule invasion. Meanwhile, we also demonstrate the potency of these NWs in practical applications by configuring them into photodetectors, which have shown reasonable response and excellent device stability.