A pinboard by
Wenxin Mao

PhD candidate, Monash University


I'm working on controlling the growth of perovskite crytals

Organic halide perovsktie is recently found to be a very good candidate to be the next generation of solar cells. The photo conversion efficiency is now approaching 23% only after 6 years study. But there are still many questions on the mechanisms of why it could work so well. My project is to study the perovskite growth mechanism and the photonic applications.


High-Temperature Ionic Epitaxy of Halide Perovskite Thin Film and the Hidden Carrier Dynamics

Abstract: High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III–V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated—a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic–inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology.

Pub.: 18 Jul '17, Pinned: 13 Sep '17

Reversible, Fast, and Wide-Range Oxygen Sensor Based on Nanostructured Organometal Halide Perovskite.

Abstract: Nanostructured materials characterized by high surface-volume ratio hold the promise to constitute the active materials for next-generation sensors. Solution-processed hybrid organohalide perovskites, which have been extensively used in the last few years for optoelectronic applications, are characterized by a self-assembled nanostructured morphology, which makes them an ideal candidate for gas sensing. Hitherto, detailed studies of the dependence of their electrical characteristics on the environmental atmosphere have not been performed, and even the effect of a ubiquitous gas such as O2 has been widely overlooked. Here, the electrical response of organohalide perovskites to oxygen is studied. Surprisingly, a colossal increase (3000-fold) in the resistance of perovskite-based lateral devices is found when measured in a full oxygen atmosphere, which is ascribed to a trap healing mechanism originating from an O2 -mediated iodine vacancies filling. A variation as small as 70 ppm in the oxygen concentration can be detected. The effect is fast (<400 ms) and fully reversible, making organohalide perovskites ideal active materials for oxygen sensing. The effect of oxygen on the electrical characteristics of organohalide perovskites must be taken into deep consideration for the design and optimization of any other perovskite-based (opto-) electronic device working in ambient conditions.

Pub.: 26 Jul '17, Pinned: 13 Sep '17

Stress-induced cubic-to-hexagonal phase transformation in perovskite nano thin films.

Abstract: The strong coupling between crystal structure and mechanical deformation can stabilize low-symmetry phases from high-symmetry phases or induce novel phase transformation in oxide thin films. Stress-induced structural phase transformation in oxide thin films has drawn more and more attention due to its significant influence on the functionalities of the materials. Here, we discovered experimentally a novel stress-induced cubic-to-hexagonal phase transformation in the perovskite nano thin films of barium titanate (BaTiO3) with a special thermomechanical treatment (TMT), where BaTiO3 nano thin films under various stresses are annealed at temperature of 575 °C. Both high resolution transmission electron microscopy and Raman spectroscopy show a higher density of hexagonal phase in the perovskite thin film under higher tensile stress. Both X-ray photoelectron spectroscopy and electron energy loss spectroscopy does not detect any change in the valance state of Ti atoms, thereby excluding the mechanism of oxygen vacancy induced cubic-to-hexagonal (c-to-h) phase transformation. First-principles calculations show that the c-to-h phase transformation can be completed by lattice shear at elevated temperature, which is consistent with the experimental observation. The applied bending plus the residual tensile stress produces shear stress in the nano thin film. The thermal energy at the elevated temperature assists the shear stress to overcome the energy barriers during the c-to-h phase transformation. The stress-induced phase transformation in perovskite nano thin films with TMT provides materials scientists and engineers a novel approach to tailor nano/microstructures and properties of ferroelectric materials.

Pub.: 27 Jul '17, Pinned: 13 Sep '17

Controlled Growth of Monocrystalline Organo-Lead Halide Perovskite and Its Application in Photonic Devices

Abstract: Organo-lead halide perovskites (OHPs) have recently emerged as a new class of exceptional optoelectronic materials, which may find use in many applications, including solar cells, light emitting diodes, and photodetectors. More complex applications, such as lasers and electro-optic modulators, require the use of monocrystalline perovskite materials to reach their ultimate performance levels. Conventional methods for forming single crystals of OHPs like methylammonium lead bromide (MAPbBr3) afford limited control over the product morphology, rendering the assembly of defined microcavity nanostructures difficult. We overcame this by synthesizing for the first time (MA)[PbBr3]⋅DMF (1), and demonstrating its facile transformation into monocrystalline MAPbBr3 microplatelets. The MAPbBr3 microplatelets were tailored into waveguide based photonic devices, of which an ultra-low propagation loss of 0.04 dB μm−1 for a propagation distance of 100 μm was demonstrated. An efficient active electro-optical modulator (AEOM) consisting of a MAPbBr3 non-linear arc waveguide was demonstrated, exhibiting a 98.4 % PL intensity modulation with an external voltage of 45 V. This novel synthetic approach, as well as the demonstration of effective waveguiding, will pave the way for developing a wide range of photonic devices based on organo-lead halide perovskites.

Pub.: 03 Jul '17, Pinned: 24 Aug '17