As electronic devices continue to scale down from the current sub-5 nm range, atomic-scale control of defects becomes increasingly crucial to suppressing their impact on the physical properties of the devices. Memristors present an excellent example of a nonlinear and dynamic device with high speed and endurance required for electronic applications ranging from neuromorphic computing to nonvolatile memories. Herein we investigate the impact of atomic defects in sub-2 nm thick MgO/Al2O3 atomic layer stack (ALS) memristors that use an M1 (switching layer)/M2 (oxygen vacancy reservoir layer) bilayer structure grown using in vacuo atomic layer deposition (iALD). Intriguingly, we revealed a direct correlation of the atomic defects in the M2 layer with the memristor dynamic behavior using combined analysis of in situ scanning tunneling spectroscopy (iSTS) on the M2 layer and ex situ characterization on the memristors. Specifically, incomplete coverage of the first ALD atomic layer of M2 on the electrode yields defects at the M2/electrode interface. Despite the monotonic increase of ALD coverage, by almost 3-fold from ∼30% to >90%, at completion of the M2 layer of ∼0.7 nm in thickness, the impact of the defects on the M2/electrode interface has been found to be detrimental to both memristor switching speed and endurance. Guided by atomistic simulation, we addressed the issue of interface defects via tuning of the Al surface hydroxylation to increase the first atomic layer ALD coverage to ∼75%, leading to improved memristor switching speed and endurance by several orders of magnitude. These findings shed light on the correlation between the atomic defects and the dynamic behavior of sub-2 nm memristors and the importance of minimizing the atomic defects in memristors for future electronic applications.

Ultrathin (1–4 nm) films of wide-bandgap semiconductors are important to many applications in microelectronics, and the film properties can be sensitively affected by defects especially at the substrate/film interface. Motivated by this, an in vacuo atomic layer deposition (ALD) was developed for the synthesis of ultrathin films of Ga2O3/Al2O3 atomic layer stacks (ALSs) on Al electrodes. It is found that the Ga2O3/Al2O3 ALS can form an interface with the Al electrode with negligible interfacial defects under the optimal ALD condition whether the starting atomic layer is Ga2O3 or Al2O3. Such an interface is the key to achieving an optimal and tunable electronic structure and dielectric properties in Ga2O3/Al2O3 ALS ultrathin films. In situ scanning tunneling spectroscopy confirms that the electronic structure of Ga2O3/Al2O3 ALS can have tunable bandgaps (Eg) between ∼2.0 eV for 100% Ga2O3 and ∼3.4 eV for 100% Al2O3. With variable ratios of Ga:Al, the measured Eg exhibits significant non-linearity, agreeing with the density functional theory simulation, and tunable carrier concentration. Furthermore, the dielectric constant ε of ultrathin Ga2O3/Al2O3 ALS capacitors is tunable through the variation in the ratio of the constituent Ga2O3 and Al2O3 atomic layer numbers from 9.83 for 100% Ga2O3 to 8.28 for 100% Al2O3. The high ɛ leads to excellent effective oxide thickness ∼1.7–2.1 nm for the ultrathin Ga2O3/Al2O3 ALS, which is comparable to that of high-K dielectric materials.

Ultrathin (sub-2 nm) Al _2 O _3 /MgO memristors were recently developed using an in vacuo atomic layer deposition (ALD) process that minimizes unintended defects and prevents undesirable leakage current. These memristors provide a unique platform that allows oxygen vacancies (V _O ) to be inserted into the memristor with atomic precision and study how this affects the formation and rupture of conductive filaments (CFs) during memristive switching. Herein, we present a systematic study on three sets of ultrathin Al _2 O _3 /MgO memristors with V _O -doping via modular MgO atomic layer insertion into an otherwise pristine insulating Al _2 O _3 atomic layer stack (ALS) using an in vacuo ALD. At a fixed memristor thickness of 17 Al _2 O _3 /MgO atomic layers (∼1.9 nm), the properties of the memristors were found to be affected by the number and stacking pattern of the MgO atomic layers in the Al _2 O _3 /MgO ALS. Importantly, the trend of reduced low-state resistance and the increasing appearance of multi-step switches with an increasing number of MgO atomic layers suggests a direct correlation between the dimension and dynamic evolution of the conducting filaments and the V _O concentration and distribution. Understanding such a correlation is critical to an atomic-scale control of the switching behavior of ultrathin memristors.

Coupling plasmons and excitons provide a promising approach to enhance the performance of photodetectors based on two-dimensional (2D) atomic layer heterostructures. Herein, we report a nanohybrid photodetector achieved by transferring a nonmetallic plasmonic WS2 nanodisk/graphene van der Waal (vdW) heterostructure grown using chemical vapor deposition, on metallic plasmonic Ag nanoparticles embedded in 20 nm thick silica (AgNP-metafilm) fabricated using in situ Ag and Si evaporation through a shadow mask. This nanohybrid photodetector enables not only superposition of the plasmonic effects from the two plasmonic nanostructures, but also the effective coupling of the plasmons and excitons in WS2 nanodisks upon illumination. This leads to a hifh responsivity of 11.7 A/W on the graphene/WS2 nanodisks/AgNP-metafilm under an incident illumination power of 5.5 x 10(-8) W at 450 nm, which represents a 500% enhancement over that of the counterpart without the AgNP-metafilm. The finite element time-domain simulation of the local light field distribution indicates that the enhancement can be attributed to enhancement of exciton (electron-hole pair) excitation and exciton-plasmon coupling in the graphene/WS2 nanodisks/AgNP-metafilm photodetectors. In addition, the approach for fabrication of the graphene/WS2 nanodisks/AgNP-metafilm heterostructures is scalable and cost efficient and hence promising for commercial applications.

Localized surface plasmon resonance (LSPR) is shown to be effective in trapping light for enhanced light absorption and hence performance in photonic and optoelectronic devices. Implementation of LSPR in all‐inorganic perovskite nanocrystals (PNCs) is particularly important considering their unique advantages in optoelectronics. Motivated by this, the first success in colloidal synthesis of AuCu/CsPbCl 3 core/shell PNCs and observation of enhanced light absorption by the perovskite CsPbCl 3 shell of thickness in the range of 2–4 nm, enabled by the LSPR AuCu core of an average diameter of 7.1 nm, is reported. This enhanced light absorption leads to a remarkably enhanced photoresponse in PNCs/graphene nanohybrid photodetectors using the AuCu/CsPbCl 3 core/shell PNCs, by more than 30 times as compared to the counterparts with CsPbCl 3 PNCs only (8–12 nm in dimension). This result illustrates the feasibility in implementation of LSPR light trapping directly in core/shell PNCs for high‐performance optoelectronics.

In this paper, the status and the challenges of utilizing MTJ based magnetic sensor for Grid sensing are reviewed. And it is shown with both modeling and experiment that an optimized MTJ based magnetic sensor can be utilized to monitor Grid current, particularly for each individual transmission line. Unlike the traditional approach, where the sensing element needs to be either integrated with, in contact with or near the transmission line, this measurement technique can be based on a contactless or a "remote" sensing setup, where the sensor is placed away from the Grid line. From the perspective of sensor, the sensitivity, the signal-to-noise ratio and the linearity of MTJ based magnetic sensor can all meet the requirement of application. It is demonstrated that an optimized DC measurement, in addition to AC measurement, can also be utilized for applications in the Grid system, such as solar energy generation.

A fully flexible strain sensor consisting of vertically aligned ZnO nanowires on graphene transferred on polyethylene terephthalate with prefabricated Au/Ti electrodes (ZnO-VANWs/Gr)/PET) has been obtained. The ZnO-VANWs were grown in solution using a seedless hydrothermal process and are single-crystalline of (0001) orientation that provides optimal piezoelectric gating on graphene when deformed mechanically. The change of the graphene channel conductance under such a piezoelectric gating through transduction of the mechanical deformation on the ZnO-VANWs/Gr was used to detect the strain induced by the deformation. Under applied normal forces of 0.30, 0.50, and 0.70 N in a dynamic manner, the ZnO-VANWs/Gr/PET strain sensors exhibited a high response and response times of ∼0.20 s to both force on and off were achieved. Under mechanical bending curvatures of 0.18, 0.23, 0.37, and 0.45 cm-1, high sensitivity of the gauge factors up to ∼248 and response times of 0.20 s/0.20 s (rise/fall) were achieved on the ZnO-VANWs/Gr/PET strain sensors. Moreover, the response changes polarity when the directions of bending alters between up and down, corresponding to the polarity change of the space charge on the ZnO-VANWs/Gr interface as a consequence of the compressive and tensile strains along the ZnO-VANWs. This result shows that the low-cost and scalable ZnO-VANWs/Gr/PET strain sensors are promising for applications in stress/strain monitoring, wearable electronics, and touch screens.

A nanohybrid piezoelectric strain sensor was fabricated by growing vertically aligned (0001)-oriented crystalline zinc oxide nanowires directly on graphene (ZnO-VANWs/Gr) using a facile seedless hydrothermal process. Under mechanical strains, the induced piezoelectric effect on the ZnO-VANWs transduces to a piezoelectric gating effect at the ZnO-VANWs/Gr interface, resulting in a modulation of the conductivity of the Gr channel through electrostatic doping. The vertical alignment of the (0001)-oriented ZnO-VANWs on Gr is ideal to achieving high strain sensitivity, and a low-defect ZnO-VANWs/Gr interface obtained in the seedless hydrothermal process is key to realizing high sensitivity and fast response. Indeed, a high sensitivity up to 3.15 X 10(-2) kPa(-1) was obtained on the ZnO-VANWs/Gr strain sensors at lower pressures of 1.1 X 10(-6)-11 Torr, together with a fast response time of similar to 0.10 s. In particular, these results represent enhancement factors of similar to 7 and 8, respectively, as compared to strain sensors of a similar structure, except having a polycrystalline ZnO seed layer on Gr for the growth of ZnO-VANWs. Therefore, our result illustrates the critical importance of the low-defect interface of the ZnO-VANWs with Gr formed in the seedless ZnO-VANW growth for realizing an optimal electrostatic gating of Gr. In addition, the ZnO-VANWs/Gr nanohybrids can be readily scaled up using the seedless hydrothermal process for commercial applications in optoelectronics and sensors.

The development of high-sensitivity magnetic field sensors at low frequencies and ambient temperatures is of great importance for many practical applications, where different aspects of the sensor performance need to be considered. In this paper, it is presented that by tuning magnetic nanostructures of the free layers in magnetic tunnel junctions, widedynamic-range or ultra-high-sensitivity tunneling magnetoresistive sensors can be obtained. Tunneling magnetoresistive sensors with a linear response from -75 mT to +75 mT are demonstrated. Also, it is demonstrated that an optimized ultra-high-sensitivity magnetic sensor with a sensitivity of 57,790 %/mT can be achieved. This sensitivity is currently the highest among all magnetoresistive sensors that have been reported. The estimated noise of our magnetic sensor is 2.3 pT/Hz1/2 at 1 Hz and 190 fT/Hz1/2 at 100 Hz respectively. This tunneling magnetoresistive sensor dissipates only 25 μW of power when it operates under an applied voltage of 1 V at room temperature.

As metal/insulator/metal tunnel junctions (MIMTJs), such as magnetic tunnel junctions and Josephson tunnel junctions, push the insulating tunnel barrier (TB) towards the ultrathin regime (<1 nm) defects inherent in current physical vapor deposition methods become a fundamental obstacle to create pinhole-free and defect-free MIMTJs. Atomic layer deposition (ALD) could offer a solution by providing a conformal, leak-free tunnel barrier with low defect density and atomic thickness as demonstrated recently in ALD Al2O3 tunnel barriers. A question arises on the viability of the ALD TBs in practical circuits of multilayer structures on which increased roughness may occur. To answer this question, this work investigates electron tunneling properties of ALD Al2O3 tunnel barriers of 1.1 –1.2 Å in thickness on half-cell MIMTJs of Al/Fe/Nb fabricated on multilayer structures of different surface roughness using in situ scanning tunneling spectroscopy. Remarkably, the tunnel barriers grown on the raised multilayer device analogue only show a moderate decrease in barrier height from 1.63 eV, to 1.51 eV and to 1.27 eV as the surface roughness increases from 0.9 nm to 2.3 nm, and to 15 nm, alongside a slight decrease in ALD coverage from ∼96%, to ∼93% and 84% on these samples. Overall, these results validate the ALD TBs of atomic thickness for future 3D arrays of devices.

All-inorganic perovskites nanostructures, such as CsPbCl3 nanocrystals (NCs), are promising in many applications including light-emitting diodes, photovoltaics, and photodetectors. Despite the impressive performance that was demonstrated, a critical issue remains due to the instability of the perovskites in ambient. Herein, we report a method of passivating crystalline CsPbCl3 NC surfaces with 3-mercaptopropionic acid (MPA), and superior ambient stability is achieved. The printing of these colloidal NCs on the channel of graphene field-effect transistors (GFETs) on solid Si/SiO2 and flexible polyethylene terephthalate substrates was carried out to obtain CsPbCl3 NCs/GFET heterojunction photodetectors for flexible and visible-blind ultraviolet detection at wavelength below 400 nm. Besides ambient stability, the additional benefits of passivating surface charge trapping by the defects on CsPbCl3 NCs and facilitating high-efficiency charge transfer between the CsPbCl3 NCs and graphene were provided by MPA. Extraordinary optoelectronic performance was obtained on the CsPbCl3 NCs/graphene devices including a high ultraviolet responsivity exceeding 106 A/W, a high detectivity of 2 × 1013 Jones, a fast photoresponse time of 0.3 s, and ambient stability with less than 10% degradation of photoresponse after 2400 h. This result demonstrates the crucial importance of the perovskite NC surface passivation not only to the performance but also to the stability of the perovskite optoelectronic devices.

The optical properties of stoichiometric iron pyrite (FeS2) nanocrystals (NCs) are characterized by strong UV-Visible (UV-Vis) absorption within the cutoff while negligible absorption beyond the cutoff in near-infrared and longer wavelengths. Herein, we show this bandgap limitation can be broken through controllable synthesis of nonstoichiometric Fe1- xS2 NCs ( x = 0.01-0.107) to induce localized surface plasmonic resonance (LSPR) absorption beyond the cutoff to short-wave infrared spectrum (SWIR, 1-3 μm) with remarkably enhanced broadband absorption across UV-Vis-SWIR spectra. To illustrate the benefit of the broadband absorption, colloidal LSPR Fe1- xS2 NCs were printed on graphene to form LSPR Fe1- xS2 NCs/graphene heterostructure photodetectors. Extraordinary photoresponsivity in exceeding 4.32 × 106 A/W and figure-of-merit detectivity D* > 7.50 × 1012 Jones have been demonstrated in the broadband of UV-Vis-SWIR at room temperature. These Fe1- xS2 NCs/graphene heterostructures are printable and flexible and therefore promising for practical optical and optoelectronic applications.

A ZnO sol-gel precursor (ZnOPr) and graphene nanoplatelets (GnPs) are mixed into a composite ink for inkjet printing photodetectors with bulk heterojunctions of ZnO/GnP on a heated SiO2/Si substrate. Heating of the SiO2/Si wafers at ∼50 °C was found optimal to prevent segregated droplets on the hydrophobic surface of the SiO2/Si substrate during printing. After printing the ZnO/GnP channels, thermal annealing at 350 °C for 2 h was performed for crystallization of ZnO and formation of the ZnO/GnP heterojunctions. The GnP concentration was varied from 0, 5, 20, and 30 mM to evaluate optimal formation of the ZnO/GnP bulk heterojunction nanocomposites based on ultraviolet photoresponse performance. The best performance was observed at the 20 mM GnP concentration with the photoresponsivity reaching 2.2 A/W at an incident ultraviolet power of 2.2 μW and a 5 V bias. This photoresponsivity is an order of magnitude better than the previously reported counterparts, including 0.13 mA/W for dropcasted ZnO-graphite composites and much higher than 0.5 A/W for aerosol printed ZnO. The improved performance is attributed to the ZnO/GnP bulk heterojunctions with improved interfaces that enable efficient exciton dissociation and the charge transport. The developed inkjet printing of sol-gel composite inks approach can be scalable and low cost for practical applications.

Pixelated broadband photoconductors consisting of chemical vapor deposition (CVD) grown graphene on Si/SiO2 and colloidal ZnO and PbS quantum dots (QDs) and FeS2 nanocrystals (NCs) are fabricated by inkjet printing of the QDs and NCs onto predefined graphene channels between Au electrodes. For a comparison, tandem devices with multilayers of QDs were also fabricated on graphene. The optoelectronic performance of these devices was characterized at different wavelengths in the ultraviolet–visible–near-infrared spectra. Specifically, the photoresponsivities are 97.5 A/W (24.4 A/W·V), 7.41 A/W (1.85 A/W·V), and 6.81 A/W (1.70 A/W·V) on the ZnO (340 nm), FeS2 (550 nm), and PbS (900 nm) channels, respectively, which correspond to the external quantum efficiencies (EQE) of 35580%, 1670%, and 940% for the three channels. In addition, the dynamic response of the ZnO-QD/graphene, PbS-QD/graphene, and FeS2-QD/graphene devices are 2 s/29 s, 300 ms/3.2 s, and 204 ms/240 ms for the rise and fall times, respectively, wh...

This work reports a general and facile route, i.e., thermal decomposition of a precursor followed by ultrafast thermal annealing (TDP-UTA), to the in situ fabrication of a nanojunction-interlinked tungsten oxide nanoparticle (WO3-NP) networks for extraordinary ultraviolet (UV) photodetection. TDP leads a spin-coated ammonium metatungstate thin layer to in situ self-assemble into a highly crystalline WO3-NP mesoporous film on SiO2/Si substrates with prepatterned electrodes. The as-synthesized WO3-NPs have dimensions comparable to the Debye length (approximate to 43 nm), which is critical to the optimal electron depletion effect for high gain in photodetection. UTA creates the NP-NP interface nanojunctions between neighboring WO3-NPs, which is the key to high-efficiency electron transport with minimized charge recombination in optoelectronic processes. The photodetectors based on such nanojunction-interlinked WO3-NP networks exhibit a photocurrent-to-dark-current ratio of 5600, the highest value for any WOx-based photodetectors ever reported. Moreover, the obtained photoresponsivity is up to 139 A/W (or 27.8 A/W.V) upon 360 nm illumination, which is over 1 order of magnitude higher than that of any previously reported WOx-nanostructure film photodetectors. These results demonstrate that the TDP-UTA route is a low-cost, robust, and scalable pathway to the in situ fabrication of interlinked semiconducting-nanostructure networks for high-performance optoelectronics and sensors.

The emerging capability to detect light over a broad spectral range is a key to technological applications in sensing, spectroscopy, imaging and communications. Colloidal semiconductor nanocrystal/graphene van der Waals heterojunctions provide a unique scheme that combines the spectral tunability and strong quantum confinement of the semiconductor nanocrystals sensitizers with superior charge mobility of graphene for extraordinary photoconductive gains. While high responsivity has been demonstrated, the spectral range is typically narrow limited by the cutoff of the semiconductor band gap of the nanocrystals. Here, a broadband photosensitizer is reported, based on doped Iron Pyrite nanocubes (FeS2 NCs) that exhibit strong localized surface plasmonic resonance (LSPR) spanning across ultraviolet through visible to near-infrared (UV-Vis-NIR). Using the printed LSPR FeS2 NCs/graphene van der Waals heterostructure, a broadband UV-Vis-NIR photoresponsivity in exceeding 1.08 x 10(6) A/W has been demonstrated through development of a ligand-exchange process to facilitate efficient charge transfer at the LSPR FeS2 NCs/graphene interface. This result demonstrates the viability of the LSPR semiconductor nanocrystal/graphene van der Waals heterostructure for high-performance broadband optoelectronics with scalability through direct printing.

Charge carrier doped iron pyrite nanocubes (FeS2 NCs) exhibit strong localized surface plasmonic resonance (LSPR) spanning across ultraviolet through visible to near-infrared (UV-Vis-NIR). Using the printed LSPR FeS2 NCs/graphene van der Waals heterostructure, broadband UV-Vis-NIR photoresponsivity in exceeding 1.1 × 106 A/W has been demonstrated by Maogang Gong, Judy Z. Wu and co-workers (article number 1701241) through development of a ligand-exchange process to facilitate efficient charge transfer at the FeS2 NCs/graphene interface.

A zinc oxide quantum dot (ZnO QD) texturing layer was printed on a SiO2 surface to resolve the issue of ink drop aggregation during inkjet printing tungsten oxide precursor (WO3Pr) ink, made from ultrasonicating ammonium metatungstate [(NH3)6H2W12O40]in a mixture of Dimethylformamide and water, for high-performance ultraviolet (UV) WO3 photoconductors. It was found that the ZnO QD textured surface offers a pinning effect of the WO3Pr ink and hence prevents the ink from aggregating and coagulating into large droplets, which prohibits formation of uniform WO3Pr films on hydrophobic SiO2/Si or glass substrates. With the ZnO QD texturing layer, a uniform WO3 film can be obtained when printing WO3Pr at room-temperature (22 °C), which reduced cracks and eliminated the coffee ring effect that arises when printing at elevated temperature such as 50 °C. High crystallinity was confirmed on the printed WO3 films, which leads to UV photoresponsivity up to 16.6 mA/W, which is comparable with the best reported on printed WO3 UV detectors without a ZnO QD texturing layer. This result shows surface texturing using a pre-printed QD layer provides a facile and compatible approach in controlling inkjet printing for high-quality oxide films directly from precursor solutions.

Heterojunction nanohybrid zinc oxide nanowires on graphene (ZnO-NW/graphene) combines crystalline ZnO-NWs with high charge mobility of graphene to provide a platform for high-performance devices.