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Purpose We conducted this meta-analysis to compare postoperative outcomes between meatal-based flap (Mathieu) and tubularized incised plate (TIP) techniques for distal hypospadias. Methods A comprehensive literature search of PUBMED, Web of Science, EMBASE, and Cochrane Library was conducted. Outcomes evaluated in this review were fistula, meatal stenosis, wound dehiscence and flap necrosis. We calculated odds ratio (OR) with 95% confidential interval (CI) to compare postoperative outcomes between Mathieu and TIP after data extraction and literature identification. All data were analyzed using Review Manager 5.2. In order to find potential affective factors, meta-regression and subgroup analyses were applied. Results 16 studies, 1386 patients, including 762 patients receiving Mathieu and 624 individuals subjected to TIP met the inclusion criteria. The synthetic data suggested that Mathieu and TIP were comparable in terms of fistula (OR = 0.93; 95% CI: 0.65 to 1.33; P = 0.70, I2 = 14%), wound dehiscence (OR = 0.89; 95% CI: 0.33 to 2.39; P = 0.81, I2 = 11%), and flap necrosis (OR = 1.9; 95% CI: 0.51 to 7.09; P = 0.20, I2 = 38%) without significant heterogeneity for each comparison group. Pooled estimates showed a significantly lower rate of meatal stenosis with Mathieu than with TIP (OR = 0.41; 95% CI: 0.24 to 0.73; P = 0.002, I2 = 4%). Subgroup analyses showed that the difference between Mathieu and TIP was more obvious in the studies published before 2013 in meatal stenosis. The modified Mathieu technique and a running suture for urethroplasty might be relevant to a lower rate of meatal stenosis in the data, although no statistical significance in the present effects model overall was found. One-way sensitivity analysis showed that the results were stable. There was no publication bias detected using both funnel plot and Egger's test. Conclusion This meta-analysis suggested that Mathieu and TIP technique were equivalent for primary distal hypospadias in terms of fistula, wound dehiscence, and flap necrosis. Pooled estimates indicated that there was a lower rate of meatal stenosis with Mathieu rather than with TIP significantly. The modified Mathieu technique and a running suture for urethroplasty might be relevant to a lower rate of meatal stenosis. Type of study Meta-analysis. Level of evidence IV

Two-dimensional materials could play an important role in beyond-CMOS (complementary metal–oxide–semiconductor) electronics, and the development of memristors for information storage and neuromorphic computing using such materials is of particular interest. However, the creation of high-density electronic circuits for complex applications is limited due to low device yield and high device-to-device variability. Here, we show that high-density memristive crossbar arrays can be fabricated using hexagonal boron nitride as the resistive switching material, and used to model an artificial neural network for image recognition. The multilayer hexagonal boron nitride is deposited using chemical vapour deposition, and the arrays exhibit a high yield (98%), low cycle-to-cycle variability (1.53%) and low device-to-device variability (5.74%). The devices exhibit different switching mechanisms depending on the electrode material used (gold for bipolar switching and silver for threshold switching), as well as characteristics (such as large dynamic range and zeptojoule-order switching energies) that make them suited for application in neuromorphic circuits. High-density memristive crossbar arrays made from two-dimensional hexagonal boron nitride can be fabricated with a yield of 98% and used to emulate artificial neural networks.

RSC Advances 12(22), 14235-14245 (2022). doi:10.1039/D2RA02456J, Published by RSC Publishing, London

Redox-based resistive random access memories (ReRAMs) are based on electrochemical processes of oxidation and reduction within the devices. The selection of materials and material combinations strongly influence the related nanoscale processes, playing a crucial role in resistive switching properties and functionalities. To date, however, comprehensive studies on device design accounting for a combination of factors such as electrodes, electrolytes, and capping layer materials related to their thicknesses and interactions are scarce. In this work, the impact of materials' configuration on interfacial redox reactions in HfO2 -based electrochemical metallization memory (ECM) and valence-change memory (VCM) systems is reported. The redox processes are studied by cyclic voltammetry, and the corresponding resistive switching characteristics are investigated. In ECM cells, the overall cell resistance depends on the electrocatalytic activity of the counter electrode. Nonetheless, the capping layer material further influences the cell resistance and the SET and RESET voltages. In VCM systems, the influence of the electrode material configuration is also pronounced, and is capable of modulating the active resistive switching interface. For both types of memory cells, the switching behavior changes significantly with variation of the oxide thickness. The results present important materials selection criteria for rationale design of ReRAM cells for various memristive applications.

Memristors have shown an extraordinary potential to emulate the plastic and dynamic electrical behaviors of biological synapses and have been already used to construct neuromorphic systems with in-memory computing and unsupervised learning capabilities; moreover, the small size and simple fabrication process of memristors make them ideal candidates for ultradense configurations. So far, the properties of memristive electronic synapses (i.e., potentiation/depression, relaxation, linearity) have been extensively analyzed by several groups. However, the dynamics of electroforming in memristive devices, which defines the position, size, shape, and chemical composition of the conductive nanofilaments across the device, has not been analyzed in depth. By applying ramped voltage stress (RVS), constant voltage stress (CVS), and pulsed voltage stress (PVS), we found that electroforming is highly affected by the biasing methods applied. We also found that the technique used to deposit the oxide, the chemical composition of the adjacent metal electrodes, and the polarity of the electrical stimuli applied have important effects on the dynamics of the electroforming process and in subsequent post-electroforming bipolar resistive switching. This work should be of interest to designers of memristive neuromorphic systems and could open the door for the implementation of new bioinspired functionalities into memristive neuromorphic systems.

Two-dimensional (2D) material-based memristors have shown several properties that are not shown by traditional ones, such as high transparency, robust mechanical strength and flexibility, superb chemical stability, enhanced thermal heat dissipation, ultralow power consumption, coexistence of bipolar and threshold resistive switching, and ultrastable relaxation when used as electronic synapse (among others). However, several electrical performances often required in memristive applications, such as the generation of multiple stable resistive states for high-density information storage, still have never been demonstrated. Here, we present the first 2D material-based memristors that exhibit three stable and well-distinguishable resistive states. By using a multilayer hexagonal boron nitride (h-BN) stack sandwiched by multilayer graphene (G) electrodes, we fabricate 5 μm × 5 μm cross-point Au/Ti/G/h-BN/G/Au memristors that can switch between each two or three resistive states, depending on the current limitation (CL) and reset voltage used. The use of graphene electrodes plus a small cross-point structure are key elements to observe the tristate operation, which has not been observed in larger (100 μm × 100 μm) devices with an identical Au/Ti/G/h-BN/G/Au structure nor in similar small (5 μm × 5 μm) devices without graphene interfacial layers (i.e., Au/Ti/h-BN/Au). Basically, we generate an intermediate state between the high resistive state and the low resistive state (LRS), named soft-LRS (S-LRS), which may be related to the formation of a narrower conductive nanofilament across the h-BN because of the ability of graphene to limit metal penetration (at low CLs). All the 2D materials have been fabricated using the scalable chemical vapor deposition approach, which is an immediate advantage compared to other works using mechanical exfoliated 2D materials.

Two dimensional (2D) materials have been used in memristors to improve and stabilize resistive switching (RS) behavior; however, most reports used large device area $(\geq 10^{4} \mu \text{m}^{2})$ , in which currents in each resistive state may be driven by largely defective regions that are not applicable in smaller devices. Here we present the first fabrication of cross-point memristors with sizes of $5 \mu \text{m} \times 5 \mu \text{m}$ using multilayer heterogeneous van der Waals structure. We have constructed graphene/hexagonal boron nitride/graphene (G/h-BN/G) devices using Ti and Au electrodes. The devices can show two-state or tristate operation depending on the current limitation (CL) used. Similar memristors with larger size of $100 \mu \text{m} \times 100 \mu \text{m}$ do not show this behavior, indicating that miniaturization of 2D materials based memristors is key to achieve this performance.

Here we show the fabrication of small-size cross point memristors using multilayer hexagonal boron nitride (h-BN) as switching layer. Excellent bipolar resistive switching (RS) with current ON/OFF ratios >1000 and low variability is demonstrated. The devices can also be operated in threshold RS mode, and the currents in high resistive state (HRS) are the lowest ever reported (10fA @0.1V), making possible their use as selector.

Memristors have recently gained growing interest due to their potential application as electronic synapses to build artificial neural networks for artificial intelligence systems. However, modulati...

The Collaborative Innovation Center of Suzhou Nano Science & Technology, the Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, and the Priority Academic Program Development of Jiangsu Higher Education Institutions are also acknowledged. Stanford co-authors acknowledge support from the Non-volatile Memory Technology Research Initiative (NMTRI). An Chen from IBM is acknowledged for revision of section 5. Alok Ranjan from Singapore University of Technology and Design is acknowledged for useful discussion on section 3.5., Resistive switching (RS) is an interesting property shown by some materials systems that, especially during the last decade, has gained a lot of interest for the fabrication of electronic devices, being electronic non-volatile memories those that have received most attention. The presence and quality of the RS phenomenon in a materials system can be studied using different prototype cells, performing different experiments, displaying different figures of merit, and developing different computational analyses. Therefore, the real usefulness and impact of the findings presented in each study for the RS technology will be also different. In this manuscript we describe the most recommendable methodologies for the fabrication, characterization and simulation of RS devices, as well as the proper methods to display the data obtained. The idea is to help the scientific community to evaluate the real usefulness and impact of an RS study for the development of RS technology., This work has been supported by the Young 1000 Global Talent Recruitment Program of the Ministry of Education of China, the National Natural Science Foundation of China (grants no. 61502326, 41550110223, 11661131002), the Jiangsu Government (grant no. BK20150343), and the Ministry of Finance of China (grant no. SX21400213).

Chemical vapor deposition (CVD) is one of the most common techniques to grow large-area hexagonal boron nitride (h-BN). However, the substrate on which the h-BN is grown plays an important role in the electrical properties of this insulating material. The high temperature used during the CVD process produces the polycrystallization of the metallic substrates, which may modify the electrical properties of the h-BN/metal from one grain to another. In this work, we compare the electrical properties of CVD-grown multilayer h-BN on three different metallic substrates: Pt, Cu and Fe. This study reveals that the properties of h-BN based devices variate remarkably from one grain to another. On the contrary, this behavior is not relevant for when using Cu and Fe. On the other hand, we have also studied the resistive switching behavior in Au/Ti/h-BN/Pt, Au/Ti/h-BN/Cu, and Au/Ag/h-BN/Fe devices, and demonstrated low variability when they are grown on the same metallic grain.

Conductive atomic force microscopy (CAFM) has been widely used for electrical characterization of thin dielectrics by applying a gentle contact force that ensures a good electrical contact without inducing additional high‐pressure related phenomena (e.g., flexoelectricity, local heat, scratching). Recently, the CAFM has been used to obtain 3D electrical images of thin dielectrics by etching their surface. However, the effect of the high contact forces/pressures applied during the etching on the electrical properties of the materials has never been considered. By collecting cross‐sectional transmission electron microscopy images at the etched regions, it is shown here that the etching process can modify the morphology of Al2O3 thin films (producing phase change, generation of defects, and metal penetration). It is also observed that this technique severely modifies the electrical properties of pSi and TiO2 wafers during the etching, and several behaviors ignored in previous studies, including i) observation of high currents in the absence of bias, ii) instabilities of etching rate, and iii) degradation of CAFM tips, are reported. Overall, this work should contribute to understand better the limitations of this technique and disseminate it among those applications in which it can be really useful.

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO2, SiOX, and n++Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle-to-cycle endurance (>1000 cycles), and low device-to-device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.