Journal of Physics D: Applied Physics

Pinki Yadav, ; Dewan, Sheetal; Mishra, Rahul; Das, Samaresh

doi: 10.1088/1361-6463/ac6635pmid: N/A

The interest in 2D layered materials based short wavelength infrared (SWIR) photodetectors (PDs) has escalated over the years with the introduction of new 2D materials showing intriguing photoresponse characteristics in the IR region. Two-dimensional materials with their mechanical flexibility, bandgap tunability, ease in exfoliation and thickness dependent optical properties have shown potential to surpass the performance of conventional, cryogenically operated semiconducting PDs. To date, a significant number of PDs have been reported using layered materials in various configurations, which have attracted the interest of research community towards scalable 2D-PDs. This review article aims to address current state-of-art SWIR PDs based on layered materials and the underlying physics. The article gives an insight into the various photodetection schemes and important figures of merit for the evaluation of PDs. The 2D materials frequently employed for designing SWIR PDs and their important characteristics are discussed in detail, with special emphasis on their photodetection range. Further, a comprehensive review of the 2D SWIR PDs based on different device structures is included, followed by a discussion on the major challenges currently prevalent in 2D SWIR PDs. Finally, the promising future prospects of 2D SWIR PDs and their important applications are described.

Chen, Juan-Nan; Wang, Qian; Lu, Hong-Ting; Zhao, Xian; Wang, Chun-Ming

doi: 10.1088/1361-6463/ac6e9dpmid: N/A

Calcium bismuth niobate (CaBi2Nb2O9, CBN) is considered to be one of the most promising high-temperature (HT) piezoelectric materials owing to its high Curie temperature of ∼940 °C, however, its low electrical resistance and poor piezoelectric properties at elevated temperatures limit its applications at high temperatures. In this work, we report the significantly enhanced dc electrical resistivity and piezoelectric performance of CBN ceramics through rare-earth Nd-substitution. The crystal structure, microstructure, and dielectric, electrical, ferroelectric and piezoelectric properties of Nd-modified CBN with nominal compositions of Ca1−xNdxBi2Nb2O9 (CBN-100xNd) have been investigated in detail. The results indicate that the substitutions of Nd3+ ions for Ca2+ ions increase the piezoelectric properties, and reduce the dielectric loss tanδ at high temperatures. The dc and ac conduction mechanisms indicate that the conduction mechanism is closely related to oxygen vacancies that are reduced through the donor substitutions of Nd3+ for Ca2+, thereby resulting in a significant improvement in the dc electrical resistivity. The optimal composition of CBN-3Nd exhibits a high piezoelectric constant d33 of 13.5 pC N−1, and a high Curie temperature Tc of 948 °C. More importantly, the CBN-3Nd exhibits good thermal stability of the electrical properties (ρ= 2.6 × 107 Ω cm at 500 °C and 1.8 × 106 Ω cm at 600 °C, kp = 6.1%–6.2% at RT ∼ 500 °C), which demonstrates that the Nd-modified CBN ceramics are promising piezoelectric materials for use in HT piezoelectric sensors.

Azuma, Hiroo

doi: 10.1088/1361-6463/ac6f2bpmid: N/A

We discuss the generation of entangled photons using a nonlinear photonic crystal and beam splitter. In our method, the photonic crystal is assumed to be composed of a material with a large second-order nonlinear optical susceptibility χ(2). Our proposal relies on two facts: (1) A nonlinear photonic crystal changes coherent incident light into squeezed light. (2) A beam splitter transforms the squeezed light into entangled light beams flying in two different directions. We estimate the yield efficiency of pairs of entangled photons per pulse of the very weak coherent light for our method at 0.0783 for a specific concrete example. Our method is more effective because the conversion efficiency (entangled biphotons per incident pump photons) in the spontaneous parametric downconversion is of the order of 4×10−6. The only drawback is that it requires very fine tuning of the frequency of signal photons fed into the photonic crystal; for example, an adjustment is given by Δν=3.11×108Hz for the signal light of ν=3.23×1014Hz. We investigate the application of entangled photons produced by our method to the BB84 quantum key distribution protocol, and we explore how to detect an eavesdropper. We suggest that our method is promising for decoy-state quantum key distribution using weak coherent light.

Talantsev, Artem; Bakhmetiev, Maxim; Morgunov, Roman

doi: 10.1088/1361-6463/ac6e13pmid: N/A

Magnetization reversal in NiFe/IrMn exchange-biased thin films was investigated under thermal cycling in an external magnetic field, applied opposite to the direction of the exchange bias field. Thermal hysteresis of magnetization accompanied by changes in magnetization polarity was observed in the applied field close to the exchange bias value. This effect appears when thermally induced variations of the exchange bias exceed the corresponding variations in coercivity. The amplitude of magnetization reversal in NiFe/IrMn structures exceeds ∼100 times the corresponding amplitude in spin-crossover molecular compounds. The observed bistability of the magnetic state, revealed by thermal hysteresis, gradually disappears with an increase in the number of cooling–heating thermal cycles, that indicates an irreversible quenching of the interfacial magnetization configuration. This effect paves the way for the creation of a new class of switching devices with thermally assisted bistability in the ferromagnetic state.

Zhang, Gongxi; Deng, Feng; Liu, Wenyuan; Shen, Shengping

doi: 10.1088/1361-6463/ac6714pmid: N/A

Electrochemical processes in solids are affected by the properties of various interfaces, where the flexoelectric effect manifests itself considerably due to the inevitable strong gradient fields. Thus, it is crucial to study the coupling between the electrochemical process and the flexoelectric effect. Based on the continuum theory, we conducted the finite element implementation for the flexochemical effect, being the coupling between flexoelectricity, Vegard effect and chemical reactions. Then, the developed method is employed to investigate the flexochemical effect arising in scanning probe microscopy (SPM), including evaluating the contributions from the flexoelectric effect and Vegard effect to the electromechanical response on material SrTiO3 (STO) in piezoresponse force microscopy (PFM) as well as to mechanical redistribution of oxygen vacancy in STO. It is found that at room temperature the nanoscale electromechanical response of the undoped STO in PFM imaging is mainly induced by the converse flexoelectricity while the contribution of direct Vegard effect is negligible. Furthermore, the contact force exerted by SPM tip in manipulating the redistribution of oxygen vacancies is multifunctional, including diminishing vacancies underneath the contact area and enriching the regions around the tip-surface contact edge and inside the sample below the tip, resulting from the synergy of the converse Vegard effect and the direct flexoelectricity. These analyses explain some experimental observations well. This paper provides a continuum framework for the analysis of electrochemomechanical systems with the flexoelectric effect.

Fan, Yuxuan; Sun, Ahui; Tian, Yuhe; Zhou, Pengchao; Niu, Yixiao; Shi, Wei; Wei, Bin

doi: 10.1088/1361-6463/ac683dpmid: N/A

The tandem organic light-emitting diodes (OLEDs) have the advantages of small current density, high current efficiency (CE), and long lifetime. We have developed the conventional and inverted tandem OLEDs using n- and p-doped planar heterojunction as a charge generation layer (CGL). The CGL consists of the bathophenanthroline:Cs2CO3 and N,N’-di-[(1-naphthalenyl)-N,N’-diphenyl]-1,1′-biphenyl)-4,4′-diamine:MoO3 bilayer structure to connect the deep blue- and deep red-emitting units. The turn-on voltage, luminescence, CE, and external quantum efficiencies of the conventional tandem OLED are 7.2 V, 5083 cd m−2, 8.45 cd A−1, and 13.94%, respectively, and the color rendering ability remains stable at a high current density of 60 mA cm−2. Moreover, the efficiency roll-off of the inverted tandem OLED is optimized to 5.5% at a luminance of 1000 cd m−2. Furthermore, a large-area (50 × 50 mm2) parallel OLEDs with a tunable red-emitting area are fabricated. The development of the OLEDs provides a new direction for the application of OLEDs in plant growth.

Liu, Yibo; Feng, Feng; Zhang, Ke; Jiang, Fulong; Chan, Ka-Wah; Kwok, Hoi-Sing; Liu, Zhaojun

doi: 10.1088/1361-6463/ac6cb4pmid: N/A

In this paper, the GaN-based green micro light-emitting diodes (Micro-LEDs) with various sizes (from 3 to 100 μm) were fabricated and electro-optically characterized. Atom layer deposition (ALD) passivation and potassium hydroxide (KOH) treatment were applied to eliminate the sidewall damage. The size dependence of Micro-LED was systematically analyzed with current-versus-voltage and current density-versus-voltage relationship. According to the favorable ideality factor results (<1.5), the optimized sidewall treatment was achieved when the device size shrank down to <10 μm. In addition, the external quantum efficiency (EQE) droop phenomenon, luminance and output power density characteristics were depicted up to the highest current density injection condition to date (120 kA cm−2), and 6 μm device exhibited an improved EQE performance with the peak EQE value of 16.59% at 20 A cm−2 and over 600k and 6M cd cm−2 at 1 and 10 A cm−2, indicating a greater brightness quality for over 3000 PPI multiple display application. Lastly, the blue shift of 6 μm device with elevating current density was observed in electroluminescence spectra and converted to CIE 1931 color space. The whole shifting track and color variation from 1 A cm−2 to 120 kA cm−2 were demonstrated by color coordinates.

Lin, Shiquan; Zheng, Mingli; Xu, Liang; Zhu, Laipan; Wang, Zhong Lin

doi: 10.1088/1361-6463/ac6f2epmid: N/A

Contact electrification (CE) has been known for over 25 centuries, but the origin of the CE remains mysterious. Recent theoretical studies suggest that flexoelectricity may drive the CE, but experimental evidence is lacking. Here, the CE between a nanoscale tip and flat polymers is studied by using atomic force microscopy. The contributions of flexoelectricity to the CE are analyzed. We focus on the effect of the load, which is coupled to the strain gradient at the contact region. It is revealed that the flexoelectric polarization in general polymers can drive electron transfer, and even reverse the intrinsic polarity of electron transfer in the CE. It implies that the flexoelectricity is one of the driving forces for the CE. The flexoelectricity induced electric field is measured by applying a voltage between the tip and the sample, which counteracts the flexocoupling voltage. Further, a band structure model is proposed, in which the surface states of the solid are suggested to be shifted by the flexoelectric polarization.

Sarikurt, Sevil; Abdullahi, Yusuf Zuntu; Durgun, Engin; Ersan, Fatih

doi: 10.1088/1361-6463/ac6e12pmid: N/A

Materials with a negative thermal expansion coefficient have diverse potential applications in electronic engineering. For instance, mixing two materials with negative and positive thermal expansion coefficients can avoid changing volume with temperature. In this study, we investigate the variation of linear thermal expansion coefficients (LTECs) of group III-Nitride monolayers (h-XN, where X = B, Al, Ga, In) with temperature using quasi-harmonic approximation. We also explore phonon thermal properties of h-XN monolayers, including specific heat, entropy, and free energy. These systems are revealed to exhibit considerably high negative LTEC values below the room temperature. To understand the origin of negative thermal expansion, we analyze the contribution of individual phonon branches to the LTEC, and it is found that the highest contribution is originating from ZA (out-of-plane acoustic) phonon mode. While h-BN and h-AlN monolayers exhibit negative LTEC values in the studied temperature range (0–800 K), unlike their bulk counterparts, the negative LTEC values converge to the zero for h-GaN and h-InN monolayers above room temperatures. These findings can be crucial in designing h-XN based nanoscale heat devices.

Jiang, Zhen; Xi, Chaoxiang; He, Guangqiang; Jiang, Chun

doi: 10.1088/1361-6463/ac6f2dpmid: N/A

The concepts of topological phases have been widely exploited in quantum optics in recent years. Here we demonstrate a topological insulator implementing topological protection of correlated biphoton states. A degenerate four-wave mixing (FWM) process of pseudospin states propagating along the topological interface is numerically simulated. Strikingly, the signal and idler photons generated from the FWM process are clarified to be entangled between two degrees of freedom—the frequencies of photon pairs and their time of arrival. The topological edge states of the pump, signal, and idler are robust against the sharp bends and defects, revealing the topological protection of energy-time entangled biphoton states. These findings could pave the way for unprecedented topological quantum devices.