Showing papers by "Yuan Gao published in 2016"

All‐Inorganic Perovskite Nanocrystals for High‐Efficiency Light Emitting Diodes: Dual‐Phase CsPbBr3‐CsPb2Br5 Composites

Abstract: A dual-phase all-inorganic composite CsPbBr3-CsPb2Br5 is developed and applied as the emitting layer in LEDs, which exhibited a maximum luminance of 3853 cd m–2, with current density (CE) of ≈8.98 cd A–1 and external quantum efficiency (EQE) of ≈2.21%, respectively. The parasite of secondary phase CsPb2Br5 nanoparticles on the cubic CsPbBr3 nanocrystals could enhance the current efficiency by reducing diffusion length of excitons on one side, and decrease the trap density in the band gap on the other side. In addition, the introduction of CsPb2Br5 nanoparticles could increase the ionic conductivity by reducing the barrier against the electronic and ionic transport, and improve emission lifetime by decreasing nonradiative energy transfer to the trap states via controlling the trap density. The dual-phase all-inorganic CsPbBr3-CsPb2Br5 composite nanocrystals present a new route of perovskite material for advanced light emission applications.

High brightness formamidinium lead bromide perovskite nanocrystal light emitting devices.

Abstract: Formamidinium lead halide (FAPbX3) has attracted greater attention and is more prominent recently in photovoltaic devices due to its broad absorption and higher thermal stability in comparison to more popular methylammonium lead halide MAPbX3. Herein, a simple and highly reproducible room temperature synthesis of device grade high quality formamidinium lead bromide CH(NH2)2PbBr3 (FAPbBr3) colloidal nanocrystals (NC) having high photoluminescence quantum efficiency (PLQE) of 55–65% is reported. In addition, we demonstrate high brightness perovskite light emitting device (Pe-LED) with these FAPbBr3 perovskite NC thin film using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) commonly known as TPBi and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM) as electron transport layers (ETL). The Pe-LED device with B3PYMPM as ETL has bright electroluminescence of up to 2714 cd/m2, while the Pe-LED device with TPBi as ETL has higher peak luminous efficiency of 6.4 cd/A and peak luminous power efficiency of 5.7 lm/W. To our knowledge this is the first report on high brightness light emitting device based on CH(NH2)2PbBr3 widely known as FAPbBr3 nanocrystals in literature.

Independent Mobility Achieved through a Wireless Brain-Machine Interface

Abstract: Individuals with tetraplegia lack independent mobility, making them highly dependent on others to move from one place to another. Here, we describe how two macaques were able to use a wireless integrated system to control a robotic platform, over which they were sitting, to achieve independent mobility using the neuronal activity in their motor cortices. The activity of populations of single neurons was recorded using multiple electrode arrays implanted in the arm region of primary motor cortex, and decoded to achieve brain control of the platform. We found that free-running brain control of the platform (which was not equipped with any machine intelligence) was fast and accurate, resembling the performance achieved using joystick control. The decoding algorithms can be trained in the absence of joystick movements, as would be required for use by tetraplegic individuals, demonstrating that the non-human primate model is a good pre-clinical model for developing such a cortically-controlled movement prosthetic. Interestingly, we found that the response properties of some neurons differed greatly depending on the mode of control (joystick or brain control), suggesting different roles for these neurons in encoding movement intention and movement execution. These results demonstrate that independent mobility can be achieved without first training on prescribed motor movements, opening the door for the implementation of this technology in persons with tetraplegia.

Unique Current-Direction-Dependent ON–OFF Switching in BiSbTeSe 2 Topological Insulator-Based Spin Valve Transistors

Abstract: In this letter, spin valve transistors are fabricated based on BiSbTeSe2 topological insulator (TI) with enhanced surface mobility ( $\sim 4039$ cm $^{2}\text{V}^{-1}\text{s}^{-1})$ . The output in our spin valve transistors exhibits a dominant steplike behavior when sweeping the magnetic field to change the magnetization orientation of the Ni21Fe79 electrode. Most importantly, the ON (low resistance)–OFF (high resistance) state can be even switched when reversing the direction of the dc current. The TI-based spin valve transistors enable the current-direction-dependent switching of ON–OFF state, allowing for the applicability in magnetic sensors and spin-logic circuits, and show the potential use of TIs as innovative current-driven spin generators.

Azimuthally Polarized, Circular Colloidal Quantum Dot Laser Beam Enabled by a Concentric Grating

Abstract: Since optical gain was observed from colloidal quantum dots (CQDs), research on CQD lasing has been focused on the CQDs themselves as gain materials and their coupling with optical resonators. Combining the advantages of a CQD gain medium and optical microcavity in a laser device is desirable. Here, we show concentric circular Bragg gratings intimately incorporating CdSe/CdZnS/ZnS gradient shell CQDs. Because of the strong circularly symmetric optical confinement in two dimensions, the output beam CQD-based circular grating distributed feedback laser is found to be highly spatially coherent and azimuthally polarized with a donut-like cross section. We also observe the strong modification of the photoluminescence spectrum by the grating structures, which is associated with modification of optical density of states. This effect confirmed the high quality of the resonator that we fabricated and the spectral overlap between the optical transitions of the emitter and resonance of the cavity. Single mode lasing...

Exciton energy recycling from ZnO defect levels: towards electrically driven hybrid quantum-dot white light-emitting-diodes

Abstract: An electrically driven quantum-dot hybrid white light-emitting diode is fabricated via spin coating CdSe quantum dots onto a GaN/ZnO nanorod matrix. For the first time, quantum dots are excited by fluorescence resonance energy transfer from the carriers trapped at surface defect levels. The prototype device exhibits achromatic emission, with a chromaticity coordinate of (0.327, 0.330), and correlated color temperature similar to sunlight.

Green Stimulated Emission Boosted by Nonradiative Resonant Energy Transfer from Blue Quantum Dots.

Abstract: Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II–VI semiconductor compound offer the potential to bridge the “green gap” in conventional semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Forster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and casc...

Device And Method For Energy Harvesting Using A Self-Oscillating Power-On-Reset Start-Up Circuit With Auto-Disabling Function

Abstract: Device and method for energy harvesting using a self-oscillating power-on reset start-up circuit. The device for energy harvesting comprises a start-up circuit for generating self-oscillation and initial boosting of an input voltage from an energy source during a start-up phase; a main boost circuit for boosting the input voltage during a steady state phase; a clock generator circuit for generating clock signals which control voltage boosting of the main boost circuit during the steady state phase; and a switching circuit coupled to the start-up circuit, the main boost circuit and the clock generator circuit for switching powering of the clock generator circuit between the start-up circuit and the main boost circuit such that the clock generator circuit is powered by only one of the start-up circuit and the main boost circuit at any point in time.

Transcriptome profiling indicating canine parvovirus type 2a as a potential immune activator

Abstract: Canine parvovirus type 2a (CPV-2a) is a variant of CPV-2, which is a highly contagious pathogen causing severe gastroenteritis and death in young dogs. However, how CPV-2 participates in cell regulation and immune response remains unknown. In this study, persistently infected MDCK cells were generated through culture passage of the CPV-2a-infected cells for ten generations. Our study showed that CPV-2a induces cell proliferation arrest and cell morphology alternation before the fourth generation, whereas, the cell morphology returns to normal after five times of passages. PCR detection of viral VP2 gene demonstrated that CPV-2a proliferate with cell passage. An immunofluorescence assay revealed that CPV-2a particles were mainly located in the cell nuclei of MDCK cell. Then transcriptome microarray revealed that gene expression pattern of MDCK with CPV-2a persistent infection is distinct compared with normal cells. Gene ontology annotation and Kyoto Encyclopedia of Genes and Genome pathway analysis demonstrated that CPV-2a infection induces a series of membrane-associated genes expression, including many MHC protein or MHC-related complexes. These genes are closely related to signaling pathways of virus-host interaction, including antigen processing and presentation pathway, intestinal immune network, graft-versus-host disease, and RIG-I-like helicases signaling pathway. In contrast, the suppressed genes mediated by CPV-2a showed low enrichment in any category, and were only involved in pathways linking to synthesis and metabolism of amino acids, which was confirmed by qPCR analysis. Our studies indicated that CPV-2a is a natural immune activator and has the capacity to activate host immune responses, which could be used for the development of antiviral strategy and biomaterial for medicine.

Stochastic domain wall depinning in permalloy nanowires with various types of notches

Abstract: Stochastic phenomena in magnetic nanowires based on domain wall (DW) motion is scientifically important thus to understand and control such behaviors are very meaningful. Here we report on the investigation of pinning and depinning of DWs in permalloy nanowires with six types of longitudinally asymmetric notches using focused magneto-optic Kerr effect (FMOKE) magnetometer and magnetic force microscopy (MFM). The hysteresis loops obtained by FMOKE indicate the generation of one or two distinct depinning fields by creating one notch close to the edge of the nanowires, in comparison multiple depinning processes occur in the nanowires with two identical notches symmetrically placed along the transverse direction, indicating more remarkable stochastic DW depinning phenomena. The MFM images verify the existence of DW in each type of nanowires and the DW sizes in the latter kind of nanowires are generally larger than those in the former ones. These observations can be explained by considering the thermal perturbation and edge or surface roughness effects in nanowires.

Biopackaging of Intracranial Pressure Microsystem for Multimodality Neuro Monitoring of Severe Head Injury Patients

Abstract: In this work, we present the biopackaging andintegration method of an intracranial pressure microsystemfor multimodal neural monitoring. The neural monitoring chipused is a System-on-Chip (SoC) which hasMicroelectromechanical Systems (MEMS) capacitive pressuresensor that is fabricated in-house after Complementary Metal-Oxide Semiconductor (CMOS) wafer fabrication process, witha temperature sensor and an oxygen sensor in order to achievemultimodal neural monitoring functions. Polyimide is used forthe substrate of the SoC die and passive component due to itsbiocompatibility and flexibility. A reversed stand-off stitchwire bonding process was employed in order to achieve a lowprofilewire looping for the interconnection of the SoC die tothe substrate. The biocompatible coatings of this implantablesystem consist of Parylene-C (for covering temperature andpressure sensors), Nafion® (for covering oxygen sensor), andmedical grade silicone elastomer (for overall deviceencapsulation). The ICP microsystem wireless reader modulewas packaged in a customized biocompatible hermetic Teflonhousing. To reinforce and seal the catheter, which is integratedwith the microsystem, during the device implantation into thebrain tissue, it is filled until the wire bonded portion withpolydimethylsiloxane. Similarly, the guiding tip of the catheterwhich will facilitate the sensor device to penetrate into thebrain tissue is made and casted from a specially designed moldusing polydimethylsiloxane as the material. Verification of thepackaging feasibility was measured by measuring thesensitivities of the pressure sensor, oxygen sensor, andtemperature sensor. The whole microsystem also passed theISO 10993-5 standards in vitro cytotoxicity test with aconclusion of no reactivity and no cell lysis cell growth in cellculture, verifying its biocompatibility

Analysis of monolithic I/Q based impedance measurement circuits: Impact of non-ideal circuit effects on accuracies

Abstract: I/Q demodulation technique is widely used in monolithic circuits for impedance measurement. The measurement accuracy of impedance is vital for various biomedical applications. The measurement accuracies of the impedance based on the I/Q demodulation was reported around 2%. This paper serves to investigate degradation of measurement accuracy due to non-ideal circuit effects, such as DC components in stimulation currents, offset of amplifiers and frequency-dependent gain in amplifier as well as angular deviation of the I/Q signals. The measurement errors due to the non-ideal effects of practical analog circuits are evaluated mathematically. The errors are quantified in terms of the real part (E r ) and imaginary part (E i ) of the impedance. From the results, we gathered that the errors in the real and imaginary parts are both proportional to gain fluctuation. With angular derivation of the I/Q signals from −9° to 9° and an impedance phase difference of 1° to 89°, E r is noted to vary from −6.6% to 4.15%. On the other hand, E i is noted to range from −896% to 894%. In circuit implementation, the measured E r is around 1%. As for the imagery part, the measurement accuracy suffers greatly, even up to 109% with just 1° angular deviation of the I/Q signals. The findings here are beneficial for impedance measurement circuitry design, providing a clear instruction on how to enhance the measurement accuracies.

A 83% peak efficiency 1.65 V to 11.4V dynamic voltage scaling supply for electrical stimulation applications in standard 0.18μm CMOS process

Abstract: In this paper, we present a dynamic voltage scaling supply (DVSS) for electrical stimulation applications. The core architecture of the proposed DVSS is based on a 7x series-parallel charge pump structure. It converts 1.65V DC input into N × 1.65V DC output in which the number of N can be dynamically scaled from 1 to 7. The DVSS circuit consumes a static power of 20μW and achieves a peak efficiency of 83.3% under a constant current load of 500μΛ at output voltage of 3 × 1.65V. The DVSS circuit has a fast response time of 146ns when switching from standby to the maximum 11.4V output. It has an intrinsic energy recycling mechanism to effectively reduce the energy wastage when switching DVSS from active to standby state. The DVSS circuit is designed and implemented in a standard 0.18μm CMOS process without high-voltage (HV) transistor option, which saves mask cost and provides possibility for HV stimulation system to be designed and integrated in more advanced technology node.

Unusual Fluorescent Properties of Stilbene Units and CdZnS/ZnS Quantum Dots Nanocomposites: White‐Light Emission in Solution versus Light‐Harvesting in Films

Abstract: Nanocomposites with organic–inorganic properties represent a new fi eld of basic research and offer prospects for many novel applications in extremely diverse fi elds, due to their remarkable emerging new properties and multifunctional nature. However, controllable manipulation of their fl uorescent properties in different phases is still challenging, which seriously limits the related applications of nanocomposites. In this work, a convenient protocol to fabricate organic–inorganic nanocomposites composed of stilbene chromophores and CdZnS/ZnS quantum dots (QDs) pairs, with controllable fl uorescent properties is presented. It is found that stable white-light emission can be achieved only in solution phase, with negligible energy transfer or reabsorption between chromophores and QDs pairs. By contrast, when the nanocomposites are deposited as blended fi lms, they cannot give rise to white-light emission, no matter what donor/acceptor volume ratios are used. However, the blended fi lms can exhibit near-unity effi ciency (94%) of Forster resonance energy transfer from QDs to chromophores. The underlying physical mechanisms are revealed through comprehensive steady-state and time-resolved spectroscopic analysis. This work suggests that the CdZnS/ZnS QDs/stilbene nanocomposites can be directly used for fl uorescence sensors and probes in biological system as well as fundamental investigation of lightharvesting, and also sheds light on developing other new materials for artifi cial photosynthesis and optoelectronics.

Colloidal quantum dot lasing for lighting and displays

Abstract: Semiconductor light-emitting diodes (LEDs) enable artificial lighting with an unprecedented level of efficiency. However, “efficiency droop” occurs in LEDs under high power injection density, practically limiting the feasible efficiency levels at high output powers. To address this problem, the concept of laser lighting has been proposed. Also, the current liquid crystal displays (LCDs) suffer the problems of low energy efficiency and small colour gamut, which can be addressed by employing the polarized white backlighting and saturated primary colours. Lasers with colloidal quantum dots (CQDs) as a gain medium can provide solutions to these limitations of current lighting and display technologies. Thus, the target of my Ph.D. thesis work is to develop and demonstrate low threshold colloidal quantum dot lasing with high linear polarization. In Chapter 2 of this thesis, unique properties of CQDs, including size dependent bandgap and discrete energy levels, which result from quantum confinement effect, are discussed. Here by adjusting the size, structure and chemical composition, CQDs that emit at various targeted wavelengths were synthesized. The resulting optical and structural characterizations are also presented. In Chapter 3, a brief review of the optical gain from CQDs is given, and the means that can be adopted to abate Auger recombination are discussed. In experimental part, CQD lasing of red, green, and blue CQDs was demonstrated. Moreover, a FRET-assisted indirect pumping scheme for CQD green lasing with standard pumping source was developed. In Chapter 4 and Chapter 5, highly polarized lasing from CQDs was demonstrated by utilization of the optical cavity effect and adoption of the polarized gain medium, respectively. The CQD DFB laser with mechanically flexible substrate was shown and analysed in Chapter 4. However, for a cylindrical optical cavity of a large diameter, which has low selectivity of TE and TM mode, the polarized gain medium that was fabricated by aligned nanorods was employed for realizing highly polarized Whispering Gallery mode lasing. These results indicate that highly polarized CQD lasing can find important uses in future lighting and displays.

A fully integrated capacitance boosting offset calibration circuit for capacitive pressure sensor

Abstract: A novel fully integrated capacitance boosting offset calibration circuit (OCC) for capacitive pressure sensor is presented. With the miniaturization of pressure sensor dimensions, the sensor sensitivity is reduced accordingly to sub-fF/mmHg range. Therefore the analog-frontend (AFE) in the sensor readout circuit has to provide very high gain to meet the sensitivity specification. The high gain of the AFE makes OCC an imperative block without which the circuit output saturates easily, leading to reduced system dynamic range. The novel capacitance boosting OCC proposed in this paper can double the dynamic range without additional chip area. Most importantly, the proposed OCC allows the decoupling of dynamic range from tuning step-size. This novel scheme allows both coarse and fine tuning of step-size 120 mV and 8 mV respectively.

Synchronous Electric Charge Extraction for low voltage Piezoelectric Energy Harvester array

Abstract: This paper introduces a topology to combine multiple low voltage Piezoelectric Energy Harvesters with the goal of widening its harvestable frequency range and improving the power output. The architecture uses a shared inductor scheme and a rectifier-free approach. The proposed harvester is able to handle multiple harvesters efficiently by harvesting the peak energy from each harvester. The proposed topology and its circuit implementation has been validated using SPICE simulation. The circuit consumes about 2.6 µA @ 3V per energy harvester and is capable of harvesting energy from low voltage sources down to 0.4V pk .

An isolated PoR based pulse generator for TEG energy harvesting with minimum startup of 150 mV and maximum series resistance of 600 Ω

Abstract: This paper presents a highly efficient 3-stage boost converter with an isolated Power-on-Reset (PoR) based starter for thermal energy harvesting. The automatic pulse generation property of the proposed PoR is coupled with a charge-pump (CP) based clock enhancer (CE) to enhance the gate-driving capability for fast and efficient boost conversion during startup. Unlike conventional PoR-based startup circuits, where the reset signals cannot be directly utilized to execute a boost conversion during startup, the proposed starter converts a chain of pulses from the PoR into level-shifted clock signals to aid direct boost conversion from sub-threshold voltages. The proposed boost converter has a minimum self-startup TEG voltage of 150 mV at the series resistance (ESR) of 450 Ω without using external devices or native MOSFET. The maximum ESR for startup is 600 Ω at the TEG voltage of 320 mV. The peak power conversion efficiency of the proposed boost converter is 78 %.

A 9.84–73.2 nJ, 0.048 mm 2 time-domain impedance sensor that provides values of resistance and capacitance

Abstract: A new time-domain impedance sensor readout circuit based on 0.18-µm CMOS technology is presented. A current DAC is used to charge the device under test (DUT) to increase the node voltage of the DUT. Using a time-domain comparator and a counter, a time period between the start of charge till the moment that the node voltage reaches a reference level is recorded and digitally converted. The resistance and capacitance components of the impedance can be quantized by using the time period data. The fabricated prototype consumes only 9.84 to 73.2 nJ of energy and requires merely 3 ms per measurement, where both are >103 times' reductions as compared to the state-of-the-arts. Moreover, to the best of the authors' knowledge, this proposed readout chip is the first of its kind that is able to deduce each resistance and capacitance component of the impedance. The chip takes up 0.048-mm2 of area.