Cavity-Suppressing Electrode Integrated with Multi-Quantum Well Emitter:A Universal Approach Toward High-Performance Blue TADF Top Emission OLED

A novel device structure for thermally activated delayed fluorescence(TADF)top emission organic light-emitting diodes(TEOLEDs)that improves the viewing angle characteristics and reduces the efficiency roll-off is presented.Furthermore,we describe the design and fabrication of a cavity-suppressing electrode(CSE),Ag(12 nm)/WO_(3)(65 nm)/Ag(12 nm)that can be used as a transparent cathode.While the TADF-TEOLED fabricated using the CSE exhibits higher external quantum efficiency(EQE)and improved angular dependency than the device fabricated using the microcavity-based Ag electrode,it suffers from low color purity and severe efficiency roll-off.These drawbacks can be reduced by using an optimized multi-quantum well emissive layer(MQW EML).The CSE-based TADF-TEOLED with an MQW EML fabricated herein exhibits a high EQE(18.05%),high color purity(full width at half maximum~59 nm),reduced efficiency roll-off(~46%at 1000 cd m^(−2)),and low angular dependence.These improvements can be attributed to the synergistic effect of the CSE and MQW EML.An optimized transparent CSE improves charge injection and light outcoupling with low angular dependence,and the MQW EML effectively confines charges and excitons,thereby improving the color purity and EQE significantly.The proposed approach facilitates the optimization of multiple output characteristics of TEOLEDs for future display applications.

Dopant-Tunable Ultrathin Transparent Conductive Oxides for Efficient Energy Conversion Devices

Ultrathin film-based transparent conductive oxides(TCOs)with a broad work function(WF)tunability are highly demanded for e cient energy conversion devices.However,reducing the film thickness below 50 nm is limited due to rapidly increasing resistance;furthermore,introducing dopants into TCOs such as indium tin oxide(ITO)to reduce the resistance decreases the transparency due to a trade-o between the two quantities.Herein,we demonstrate dopant-tunable ultrathin(≤50 nm)TCOs fabricated via electric field-driven metal implantation(m-TCOs;m=Ni,Ag,and Cu)without com-promising their innate electrical and optical properties.The m-TCOs exhibit a broad WF variation(0.97 eV),high transmittance in the UV to visible range(89–93%at 365 nm),and low sheet resistance(30–60Ωcm-2).Experimental and theoretical analyses show that interstitial metal atoms mainly a ect the change in the WF without substantial losses in optical transparency.The m-ITOs are employed as anode or cathode electrodes for organic light-emitting diodes(LEDs),inorganic UV LEDs,and organic photovoltaics for their universal use,leading to outstanding performances,even without hole injection layer for OLED through the WF-tailored Ni-ITO.These results verify the proposed m-TCOs enable e ective carrier transport and light extraction beyond the limits of traditional TCOs.

Ag-GST/HfO_(x)-based unidirectional threshold switching selector with low leakage current and threshold voltage distribution for high-density cross-point arrays

The use of electrochemical-metallization-based volatile threshold switching selectors in cross-point arrays has been widely explored owing to their high on-off ratios and simple structure.However,these devices are unsuitable for cross-point architectures because of the difficulty in controlling the random filament formation that results in large fluctuations in the threshold voltage during operation.In this study,we investigated the unidirectional threshold transition characteristics associated with an Ag/GST/HfO_(x)/Pt-based bilayer selector and demonstrated the occurrence of a low leakage current(<1×10^(-11) A) and low distribution of the threshold voltage(Δ0.11 V).The bilayer structure could control the filament formation in the intermediate state through the insertion of an HfO_(x) tunneling barrier.By stacking a bilayer selector with NiO_(x)based resistive random-access memory,the leakage and programming currents of the device could be significantly decreased.For the crossbar array configuration,we performed equivalent circuit analysis of a one-selector oneresistor(1S1R) devices and estimated the optimal array size to demonstrate the applicability of the proposed structure.The maximum acceptable crossbar array size of the 1S1R device with the Ag/GST/HfO_(x)/Pt/Ti/NiO_(x)/Pt structure was 5.29×10^(14)(N^(2),N=2.3×10^(7)).

2D Ti_(3)C_(2)T_(x)MXene-derived self-assembled 3D TiO_(2)nanoflowers for nonvolatile memory and synaptic learning applications

Two-dimensional(2D)semiconducting materials and transition-metal oxides are promising materials for nonvolatile memory and brain-inspired neuromorphic computing applications.However,it remains chal-lenging to obtain high-quality stacked 2D films with low energy consumptions(or drive currents)be-cause of their high interfacial resistance.In this study,we synthesized 2D Ti_(3)C_(2)T_(x)MXene-derived three-dimensional(3D)TiO_(2)nanoflowers(NFs)as a feasible resistive switching(RS)material with outstanding electronic properties and synaptic learning capabilities.The electrical and optical characteristics of the synthesized material were determined through density functional theory calculations.Electrical measure-ments of the Al/Ti_(3)C_(2)T_(x)-TiO_(2)NF/Pt memory device indicated the occurrence of forming-free switching phenomena with extremely low switching voltages(0.68-0.53 V),stable ON/OFF ratio(2.3×103),and retention greater than 105 s.The Holt-Winters exponential smoothing technique was used for mod-eling and predicting the switching voltages of the RS device.The mechanism underlying the reliable RS was confirmed by observing the dense conductive filaments through conductive atomic force mi-croscopy.Interestingly,the 2D Ti_(3)C_(2)T_(x)MXene-derived 3D TiO_(2)NF-based RS device mimicked the po-tentiation/depression and spike-time-dependent plasticity of a biological synapse.Finally,a convolutional neural network was implemented based on the observed synaptic weights of Al/Ti_(3)C_(2)T_(x)-TiO_(2)NF/Pt for image-edge detection.

Dual-frequency piezoelectric micromachined ultrasound transducer based on polarization switching in ferroelectric thin films

Due to its additional frequency response,dual-frequency ultrasound has advantages over conventional ultrasound,which operates at a specific frequency band.Moreover,a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution.This facilitates the availability of more advanced techniques that simultaneously require low-and high-frequency ultrasounds,such as harmonic imaging and image-guided therapy.In this study,we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer(PMUT).Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging.To switch the ultrasound frequency,the only requirement is to tune a DC bias to control the polarization state of the ferroelectric film.Flextensional vibration of the PMUT membrane strongly depends on the polarization state,producing low-and high-frequency ultrasounds from a single excitation frequency.This strategy for dual-frequency ultrasounds meets the requirement for either multielectrode configurations or heterodesigned elements,which are integrated into an array.Consequently,this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.

Manipulation of blue TADF top-emission OLEDs by the first-order optical cavity design:toward a highly pure blue emission and balanced charge transport

The broad luminescence spectrum of a thermally activated delayed fluorescence(TADF)organic light-emitting diode(OLED)is a critical issue to overcome for its application in high-color-purity displays.Herein,a novel device structure that utilizes the first-order microcavity optical mode with a high radiance intensity is demonstrated to solve this problem by considering the charge transport properties through the analysis of hole-only and electron-only devices.In addition,by tuning the optical interference near the semitransparent top cathode layers consisting of thin silver and organic capping layers,light extraction is increased by nearly 2 times compared to the device without a capping layer.Consequently,the optimized blue TADF top-emission OLED exhibits much lower full width at half-maximum,higher maximum current efficiency,and external quantum efficiency compared to the device before optimization.This approach is expected to provide a simple but effective way to further enhance the spectral purity of the conventional TADF-based OLEDs.

Enhanced light emission from AlGaN/GaN multiple quantum wells using the localized surface plasmon effect by aluminum nanoring patterns

We investigate the localized surface plasmon(LSP) effect by Al nanorings on the AlGaN/GaN multiple quantum well(MQW) structure emitting at 365 nm. For this experiment, first, the size of Al nanorings is optimized to maximize the energy transfer(or coupling) between the LSP and MQW using the silica nanospheres. Then, the Al nanorings with an outer diameter of 385 nm, which exhibit a strong absorption peak in the near-ultraviolet region, are applied to the top surface of the AlGaN/GaN MQW. The photoluminescence(PL) intensity of the MQW structure with Al nanorings increased by 227% at 365 nm compared to that without Al nanorings.This improvement is mainly attributed to an enhanced radiative recombination rate in the MQWs through the energy-matched LSPs by the temperature-dependent PL and time-resolved PL analyses. The radiative lifetime was about two times shorter than that of the structure without Al nanorings at room temperature. In addition, the measured PL efficiency at room temperature of the structure with Al nanorings was 33%, while that of the structure without Al nanorings was 19%, implying that LSP-QW coupling together with the nanoring array pattern itself played important roles in the enhancement.

Electrochemically metal-doped reduced graphene oxide films:Properties and applications

The fine control of doping levels in graphene materials such as reduced graphene oxide(RGO)is important to properly manipulate their ambipolar transport characteristics for various device applications.However,conventional doping methods involve complex chemical reactions,large-scale doping processes,and poor stability.Herein,a simple and controllable electrochemical doping treatment(EDT),performed via the conductive channels created at the RGO surface by the application of an electric field,is introduced to tailor the electrical properties of RGO films.X-ray photoelectron spectroscopy and Raman spectroscopy measurements are performed to detect the presence of Ni atoms in RGO films after the EDT(EDT-RGO).Then,EDT-RGO field-effect transistors(FETs)are fabricated with different doping areas(0 to 100%fractional area)on the RGO active channel to investigate the effect and selective-area doping capability of the EDT.Owing to p-type doping compensation by the intercalated Ni atoms,the electron mobility of the EDT-RGO FET decreases from 1.40 to 0.12 cm2 V-1s-1 compared with that of the undoped RGO-FET,leading to the conversion from ambipolar to unipolar p-type transfer characteristics.

Multilevel resistive switching and synaptic plasticity of nanoparticulated cobaltite oxide memristive device

Multilevel resistive switching(RS)is a key property to embrace the full potential of memristive devices for non-volatile memory and neuromorphic computing applications.In this study,we employed nanoparticulated cobaltite oxide(Co_(3)O_(4))as a model material to demonstrate the multilevel RS and synaptic learning capabilities because of its multiple and stable redox state properties.The Pt/Co_(3)O_(4)/Pt memristive device exhibited tunable RS properties with respect to different voltages and compliance currents(CC)without the electroforming process.That is,the device showed voltage-dependent RS at a higher CC whereas CC-dependent RS was observed at lower CC.The device showed four different resistance states during endurance and retention measurements and non-volatile memory results indicated that the CC-based measurement had less variation.Besides,we investigated the basic and complex synaptic plasticity properties using the analog current-voltage characteristics of the Pt/Co_(3)O_(4)/Pt device.In particular,we mimicked the potentiation–depression and four-spike time-dependent plasticity(STDP)rules such as asymmetric Hebbian,asymmetric anti-Hebbian,symmetric Hebbian,and symmetric antiHebbian learning rules.The results of the present work indicate that the cobaltite oxide is an excellent nanomaterial for both multilevel RS and neuromorphic computing applications.