Negative differential resistance behavior in oxide memristors, especially those using NbO2, is gaining renewed interest because of its potential utility in neuromorphic computing. However, there has been a decade-long controversy over whether the negative differential resistance is caused by a relatively low-temperature non-linear transport mechanism or a high-temperature Mott transition. Resolving this issue will enable consistent and robust predictive modeling of this phenomenon for different applications. Here we examine NbO2 memristors that exhibit both a current-controlled and a temperature-controlled negative differential resistance. Through thermal and chemical spectromicroscopy and numerical simulations, we confirm that the former is caused by a ~400 K non-linear-transport-driven instability and the latter is caused by the ~1000 K Mott metal-insulator transition, for which the thermal conductance counter-intuitively decreases in the metallic state relative to the insulating state.

Transition metal oxide memristors, or resistive random-access memory (RRAM) switches, are under intense development for storage-class memory because of their favorable operating power, endurance, speed, and density. Their commercial deployment critically depends on predictive compact models based on understanding nanoscale physicochemical forces, which remains elusive and controversial owing to the difficulties in directly observing atomic motions during resistive switching, Here, using scanning transmission synchrotron x-ray spectromicroscopy to study in-situ switching of hafnium oxide memristors, we directly observed the formation of a localized oxygen-deficiency-derived conductive channel surrounded by a low-conductivity ring of excess oxygen. Subsequent thermal annealing homogenized the segregated oxygen, resetting the cells towards their as-grown resistance state. We show that the formation and dissolution of the conduction channel are successfully modeled by radial thermophoresis and Fick diffusion of oxygen atoms driven by Joule heating. This confirmation and quantification of two opposing nanoscale radial forces that affect bipolar memristor switching are important components for any future physics-based compact model for the electronic switching of these devices.Keywords: Memristors, thermophoresis, operating mechanism, oxygen migration, filament.The recent surge in technological and commercial interest in transition-metal-oxide memristors, especially those utilizing hafnium oxide as the switching material, is accompanied by urgent efforts to formulate a compact predictive model of their behavior in large-scale integrated circuits. 1-9 Several efforts in this direction include first-principles and analytical modeling, 8,10,11 materials characterization, 2,12,13 and circuit characterization and modeling. 14,15 The resultant models are incomplete and controversial owing to a lack of understanding of the nanoscale physico-chemical forces that determine atomic motions during switching, particularly with regard to the presence and sign of temperaturegradient-driven thermophoresis of oxygen atoms, and quantification of the concentration-gradient-driven Fick diffusion. 7,8,11,16,17 Direct in-situ and in-operando studies of localized atomic motion during memristor switching can resolve these issues and improve our modeling, but such observations face steep experimental challenges due to the extremely high resolution and sensitivity required to detect atomic motions inside a functioning cell. 2,4,18,19 In order to non-destructively study the chemical and position changes associated with oxygen atoms during memristor operation, we utilized a synchrotron-based scanning transmission x-ray microscopy (STXM) system tuned to the O K-edge with a spatial resolution of <31 nm and a spectral resolution of ~70 meV. 20 We analyzed a prototype device that had only one oxide layer to permit an unambiguous analysis of the results. To enable x-ray transmission experiments, operational memristor cells for this study wer...

Metal-oxide memristors, or resistive random access memory (RRAM) switches, in particular utilizing HfO x as the resistive switching material, have seen significant interest recently for nonvolatile memory and computation applications. [1][2][3][4][5] There has been particular interest in understanding the role of migration of oxygen atoms in determining the operation of memristors. 6-11 Similar recent advances in understanding the localized nanoscale physico-chemical changes underlying resistance switching 4,12-15 have opened up fresh interests into studying the effect of atomic movements on extended device operation and the nanoscale material behavior during eventual failure and possible techniques to mitigate such failure. [16][17][18] To enable scanning transmission x-ray microscopy (STXM) measurements, each device was built on a 200 nm low-stress Si 3 N 4 film suspended over 50 µm x 50 µm holes etched through a silicon substrate. 13 We fabricated crosspoint HfO x devices with an active area of 2 µm x 2 µm ( Figure 1a) by depositing a bottom electrode (15 nm Pt), a blanket layer of 6 nm HfO 2 , followed by the top electrode (10 nm TiN and 15 nm Pt). Typical currentvoltage plots of these devices (Figure 1b) exhibited the well-recognized resistance switching behavior, or pinched hysteresis loop, that characterizes a memristor. 19 During operation, high and low non-volatile resistance states (also called OFF and ON, respectively) were repeatedly accessed using bipolar voltage pulses. STXM experiments were performed using resonantly tuned x-ray beams mostly in the O K-edge region, with spectral resolution of ~70 meV and a beam diameter <30 nm. 20 The device was electrically connected inside the chamber of the system to enable in-situ operation and ON/OFF cycling to emulate ageing of the memristor. The x-ray absorption spectrum of the material stack within a device crosspoint before its operation (Figure 2a) revealed oxygen bonds to both Hf and Ti, suggesting oxidation of Ti upon sputter deposition of TiN onto HfO 2 and a resulting mixture of Ti and Hf oxides. We used the absorption of the pre O K-edge at 522 eV to monitor total thickness and other structural modulations (especially electrode distortions), the intensity of the 531 eV peak (the lowest conduction band of the stack) as an indicator of the relative conductivity within the crosspoint, 1,21 and the post O K-edge at 570 eV to determine the local oxygen concentration in the film.

SUMMARYBrain dysfunction in erectile dysfunction (ED) has been identified by multiple neuroimaging studies. A recent MRI study indicated grey matter alterations in ED patients. This study aims to investigate the microstructural changes of cerebral white matter (WM) in psychological ED patients and their possible correlations with clinical variables. Twenty-seven psychological ED patients and 27 healthy subjects (HS) were included and underwent a magnetic resonance (MR) diffusion tensor imaging (DTI) scan. The tract-based spatial statistics were employed to identify the WM structure alterations in psychological ED patients. The multiple DTI-derived indices′ [fractional anisotropy (FA), axial diffusivity (AD) and mean diffusivity (MD)] correlations with the symptoms and their durations, respectively, were analysed. The IIEF-5, quality of erection questionnaire (QEQ) and the self-esteem and relationship (SEAR) questionnaire were used to assess the symptoms of psychological ED patients. Compared with HS, the psychological ED patients showed increased FA values, reduced MD values and reduced AD values in multiple WM tracts including the corpus callosum (genu, body and splenium), corticospinal tract, internal capsule, corona radiata, external capsule and superior longitudinal fasciculus (p < 0.05, threshold-free cluster enhancement corrected). Both of the IIEF scores and QEQ scores of ED patients showed a significantly negative correlation with the average FA values, and positive correlation with average AD values and MD values in the splenium of the corpus callosum (p < 0.05). The results provided preliminary evidence of WM microstructural changes in patients with psychological ED. The morphological alterations in the splenium of the corpus callosum were related to the symptom severity.

In this paper, we reported the development of a highly sensitive and selective resonance light scattering (RLS) technique for glutathione using gold nanoparticle probes. The assay relies upon the distance-dependent optical properties of gold nanoparticles, the self-assembly of glutathione on gold nanoparticles, and the interaction of a 2 : 1 glutathione-Cu(2+) complex. In the presence of Cu(2+), glutathione could rapidly induce the aggregation of gold nanoparticles, thereby resulting in greatly enhanced RLS intensity and red-to-blue (or purple) color change. The concentration of glutathione can be determined by the naked eye or a fluorescence spectrometer. Under the optical conditions, the detection of glutathione can be finished within 20 min, and the detection limit of 10 nM can be reached. The concentration range of the probe is 40-280 nM. The proposed method holds a specific selectivity toward glutathione and it is applied to the detection of glutathione in human serum with satisfactory results. In addition, the assay shows great potential application for disease-associated biomarkers, and it will meet the great demand for amino acid determination in fields such as food processing, biochemistry, pharmaceutical, and clinical analysis.