RSS 1.0Material Science and Engineering Program
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Recent Submissions
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Suppressed phase segregation for triple-junction perovskite solar cells(Nature, Springer Science and Business Media LLC, 2023-03-28) [Article]The tunable band gaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics1,2. However, light-induced phase segregation limits their efficiency and stability3-5: this occurs in wide band gap (> 1.65 eV) I/Br mixed perovskite absorbers, and becomes even more acute in the top cells of triple-junction solar photovoltaics that requires a fully 2.0 eV band gap absorber2,6. We report herein that lattice distortion in I/Br mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion migration energy barrier arising from the decreased average interatomic distance between A-site cation and iodide. Using a ~2.0 eV Rb/Cs mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3% (23.3% certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 V. This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80% of their initial efficiency following 420 hours of operation at the maximum power point.
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Nitrogen-Based Magneto-Ionic Manipulation of Exchange Bias in CoFe/MnN Heterostructures(Accepted by ACS Nano, 2023-03-27) [Article]Electric field control of the exchange bias effect across ferromagnet/antiferromagnet (FM/AF) interfaces has offered exciting potentials for low-energy-dissipation spintronics. In particular, the solid state magneto-ionic means is highly appealing as it may allow reconfigurable electronics by transforming the all-important FM/AF interfaces through ionic migration. In this work, we demonstrate an approach that combines the chemically induced magneto-ionic effect with the electric field driving of nitrogen in the Ta/Co0.7Fe0.3/MnN/Ta structure to electrically manipulate exchange bias. Upon field-cooling the heterostructure, ionic diffusion of nitrogen from MnN into the Ta layers occurs. A significant exchange bias of 618 Oe at 300 K and 1484 Oe at 10 K is observed, which can be further enhanced after a voltage conditioning by 5% and 19%, respectively. This enhancement can be reversed by voltage conditioning with an opposite polarity. Nitrogen migration within the MnN layer and into the Ta capping layer cause the enhancement in exchange bias, which is observed in polarized neutron reflectometry studies. These results demonstrate an effective nitrogen-ion based magneto-ionic manipulation of exchange bias in solidstate devices.
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Hybrid 2D/CMOS microchips for memristive applications(Nature, Springer Science and Business Media LLC, 2023-03-27) [Article]Exploiting the excellent electronic properties of two-dimensional (2D) materials to fabricate advanced electronic circuits is a major goal for the semiconductors industry1-2. However, most studies in this field have been limited to the fabrication and characterization of isolated large (>1µm2) devices on unfunctional SiO2/Si substrates. Some studies integrated monolayer graphene on silicon microchips as large-area (>500µm2) interconnection3 and as channel of large transistors (~16.5µm2)4-5, but in all cases the integration density was low, no computation was demonstrated, and manipulating monolayer 2D materials was challenging because native pinholes and cracks during transfer increase variability and reduce yield. Here we present the fabrication of high-integration-density 2D/CMOS hybrid microchips for memristive applications — CMOS stands for complementary metal oxide semiconductor. We transfer a sheet of multilayer hexagonal boron nitride (h-BN) onto the back-end-of-line (BEOL) interconnections of silicon microchips containing CMOS transistors of the 180nm node, and finalize the circuits by patterning the top electrodes and interconnections. The CMOS transistors provide outstanding control over the currents across the h-BN memristors, which allows us to achieve endurances of ~5 million cycles in memristors as small as ~0.053µm2. We demonstrate in-memory computation by constructing logic gates, and measure spike-timing dependent plasticity (STDP) signals that are suitable for the implementation of spiking neural networks (SNN). The high performance and the relatively-high technology readiness level achieved represent a significant advance towards the integration of 2D materials in microelectronic products and memristive applications.
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Thickness-tunable magnetic and electronic transport properties of the quasi-two-dimensional van der Waals ferromagnet Co0.27TaS2 with disordered intercalation(Accepted by Physical Review B, American Physical Society, 2023-03-27) [Article]The intercalation of magnetic elements in non-magnetic van der Waals (vdW) materials is an effective way to design novel (quasi) 2D magnets and produce exotic properties. More specifically, how exactly the intercalator is distributed within the synthetic crystal can also affect the physical properties substantially. In contrast to conventional 3d transition-metal intercalates of niobium and tantalum dichalcogenides, which commonly have 2 × 2- or Ö3 × Ö3- type ordered intercalation, we report a disordered intercalation of Co atoms between the vdW gaps of 2H-tantalum disulfide (2H-TaS2). The obtained quasi-vdW ferromagnet Co0.27TaS2 shows both perpendicular magnetic anisotropy and thickness-tunable magnetic properties. More interestingly, the temperature dependence of electrical resistivity shows a semiconductor-like behavior, in contrast to the metallic feature of other analogs in this material family. This unexpected phenomenon can be understood through a variable-range hopping mechanism, which is due to highly disordered intercalation. Moreover, Co0.27TaS2 shows a side-jump scattering dominated anomalous Hall effect, which can also be related to the disordered distribution of Co intercalators.
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Flexible Oxide Thin Film Transistors, Memristors, and Their Integration(Advanced Functional Materials, Wiley, 2023-03-26) [Article]Flexible electronics have seen extensive research over the past years due to their potential stretchability and adaptability to non-flat surfaces. They are key to realizing low-power sensors and circuits for wearable electronics and Internet of Things (IoT) applications. Semiconducting metal-oxides are a prime candidate for implementing flexible electronics as their conformal deposition methods lend themselves to the idiosyncrasies of non-rigid substrates. They are also a major component for the development of resistive memories (memristors) and as such their monolithic integration with thin film electronics has the potential to lead to novel all-metal-oxide devices combining memory and computing on a single node. This review focuses on exploring the recent advances across all these fronts starting from types of suitable substrates and their mechanical properties, different types of fabrication methods for thin film transistors and memristors applicable to flexible substrates (vacuum- or solution-based), applications and comparison with rigid substrates while additionally delving into matters associated with their monolithic integration.
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Efficient Near-Infrared Electroluminescence from Lanthanide-Doped Perovskite Quantum Cutters(Angewandte Chemie (International ed. in English), Wiley, 2023-03-25) [Article]Perovskite nanocrystals (PeNCs) deliver size and composition-tunable luminescence of high efficiency and color purity in the visible. However, attaining efficient electroluminescence (EL) in the near-infrared (NIR) region from PeNCs is challenging, limiting their potential applications. Here we demonstrate a highly efficient NIR light emitting diode (LED) by doping ytterbium ions into the PeNCs host (Yb3+:PeNCs), extending the EL wavelengths toward 1000 nm, which is achieved through a direct sensitization of Yb3+ ions by the PeNC host. Efficient quantum cutting processes enable high photoluminescence quantum yields (PLQYs) of up to 126% from the Yb3+:PeNCs. Through halide-composition engineering and surface passivation strategy to improve both PLQY and charge transport balance, we demonstrate an efficient NIR LED with a peak EQE of 7.7% at a central wavelength of 990 nm, representing the most efficient perovskite-based LEDs with emission wavelengths beyond 850 nm.
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Editorial Special Issue on Dielectrics for 2-D Electronics(IEEE Transactions on Electron Devices, Institute of Electrical and Electronics Engineers (IEEE), 2023-03-24) [Article, Editorial]It is our great pleasure to introduce this Special Issue on Dielectrics for 2-D Electronics to the IEEE TRANSACTIONS ON ELECTRON DEVICES readership. This Special Issue features the latest research aiming to clarify which would be the most suitable dielectric materials for state-of-the-art electronic devices containing 2-D materials.
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Ferroelectricity in layered bismuth oxide down to 1 nanometer(Science, American Association for the Advancement of Science (AAAS), 2023-03-24) [Article]Atomic-scale ferroelectrics are of great interest for high-density electronics, particularly field-effect transistors, low-power logic, and nonvolatile memories. We devised a film with a layered structure of bismuth oxide that can stabilize the ferroelectric state down to 1 nanometer through samarium bondage. This film can be grown on a variety of substrates with a cost-effective chemical solution deposition. We observed a standard ferroelectric hysteresis loop down to a thickness of ~1 nanometer. The thin films with thicknesses that range from 1 to 4.56 nanometers possess a relatively large remanent polarization from 17 to 50 microcoulombs per square centimeter. We verified the structure with first-principles calculations, which also pointed to the material being a lone pair–driven ferroelectric material. The structure design of the ultrathin ferroelectric films has great potential for the manufacturing of atomic-scale electronic devices.
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Giant Nonlinear Optical Response via Coherent Stacking of In-Plane Ferroelectric Layers(Advanced Materials, Wiley, 2023-03-23) [Article]Thin ferroelectric materials hold great promise for compact nonvolatile memory, nonlinear optical and optoelectronic devices. Herein, we report an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect: group-IV monochalcogenide SnSe. Nanometer-scale ferroelectric domains with ∼90°/270° twin boundaries or ∼180° domain walls are revealed in physical vapor deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals (vdW) ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second harmonic fields, and the as-resulted second-harmonic generation was observed to be 100 times more intense than monolayer WS2. This work demonstrates in-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.
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Zero-dimensional Cu(i)-based organometallic halide with green cluster-centred emission for high resolution X-ray imaging screens(Chemical Communications, Royal Society of Chemistry (RSC), 2023-03-22) [Article]In this communication, we report a low-dimensional perovskite-related structure based on Cu(I) organometallic halide with strong green cluster-centred emission and near-unity photoluminescence quantum yield. The 0D [Rb(18-crown-6)]2Cu4I6 was sucessfully applied for X-ray imaging screens which exhibit high spatial resolution of 16.8 lp mm−1 and low detection limit of 458 nGy s−1.
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Zero-dimensional Cu(i)-based organometallic halide with green cluster-centred emission for high resolution X-ray imaging screens(Chemical Communications, Royal Society of Chemistry (RSC), 2023-03-22) [Article]In this communication, we report a low-dimensional perovskite-related structure based on Cu(I) organometallic halide with strong green cluster-centred emission and near-unity photoluminescence quantum yield. The 0D [Rb(18-crown-6)]2Cu4I6 was sucessfully applied for X-ray imaging screens which exhibit high spatial resolution of 16.8 lp mm−1 and low detection limit of 458 nGy s−1.
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Observation of cnoidal wave localization in nonlinear topolectric circuits(Physical Review Research, American Physical Society (APS), 2023-03-21) [Article]We observe a localized cnoidal (LCn) state in an electric circuit network. Its formation derives from the interplay of nonlinearity and the topology inherent to a Su-Schrieffer-Heeger (SSH) chain of inductors. Varicap diodes act as voltage-dependent capacitors, and create a nonlinear on-site potential. For a sinusoidal voltage excitation around midgap frequency, we show that the voltage response in the nonlinear SSH circuit follows the Korteweg-de Vries equation. The topological SSH boundary state, which relates to a midgap impedance peak in the linearized limit is distorted into the LCn state in the nonlinear regime, where the cnoidal eccentricity decreases from edge to bulk.
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Visualization of Surface Charge Carrier Diffusion Lengths in Different Perovskite Crystal Orientations Using 4D Electron Imaging(Advanced Optical Materials, Wiley, 2023-03-20) [Article]Understanding charge carrier dynamics on the surface of materials at the nanometer and femtosecond scales is one of the key elements to optimizing the performance of light-conversion devices, including solar cells. Unfortunately, most of the pump-probe characterization techniques are surface-insensitive and obtain information from the bulk due to the large penetration depth of the pulses. However, ultrafast scanning electron microscopy (USEM) is superior in visualizing carrier dynamics at the surface with high spatial-temporal resolution. Here, the authors successfully used USEM to uncover the tremendous effect of surface orientations and termination on the charge carrier of MAPbI3 perovskite single crystals. Time-resolved secondary electrons snapshots and density functional theory calculations clearly demonstrate that charge carrier diffusion, surface trap density, surface work function, and carrier concentration are strongly facet-dependent. The results display a diffusion length of 22 micrometers within 6.0 nanoseconds along (001) orientation. While (100) facet forms defect states that prevent carrier diffusion and shows an increase in the surface work function leading to dark contrast and fast charge carrier recombination. These findings provide a new key component to optimizing the surface of perovskites, thus paving the way for even more efficient and stable solar-cell devices based on perovskite single crystals.
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Engineering grain boundaries in monolayer molybdenum disulfide for an efficient water/ion separation(Research Square Platform LLC, 2023-03-20) [Preprint]Atomically thin two-dimensional (2D) materials have long been considered as ideal platforms for developing separation membranes. However, it is difficult to generate uniform subnanometer pores over large areas on 2D materials. Herein, we report that the well-defined defect structure of monolayer MoS2, namely, eight-membered ring (8-MR) pores typically formed at the boundaries of two antiparallel grains, can serve as molecular sieves for efficient water/ion separation. The 8-MR pores (4.2 × 2.4 Å) in monolayer MoS2 allow rapid single-file water transport while rejecting various hydrated ions. Further, the density of grain boundaries and, consequently, the density of pores can be tuned by regulating the nucleation density and size of MoS2 grains during the chemical vapor deposition process. The optimized MoS2 membrane exhibited an ultrahigh water/NaCl selectivity of ~6.5 × 104 at a water permeance of 232 mol m−2 h−1 bar−1, outperforming the state-of-the-art desalination membranes. When used for direct hydrogen production from seawater by combining the forward osmosis and electrochemical water splitting processes, the membrane achieved ~40 times the energy conversion efficiency of commercial polymeric membranes. It also exhibited a rapid and selective proton transport behavior desirable for fuel cells and electrolysis. The bottom-up approach of creating precise pore structures on atomically thin films via grain boundary engineering presents a promising route for producing large-area membranes suitable for various applications.
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Flexible self-powered DUV photodetectors with high responsivity utilizing Ga2O3/NiO heterostructure on buffered Hastelloy substrates(Applied Physics Letters, AIP Publishing, 2023-03-20) [Article]In this research, β-Ga2O3/NiO heterostructures were grown directly on CeO2 buffered Hastelloy flexible substrates. With pulsed laser deposition under high temperatures, as-grown β-Ga2O3 and NiO thin films have a preferred out-of-plane orientation along the ⟨−201⟩ and ➎111➉ directions. This is due to the ideal epitaxial ability of the CeO2 buffer layer, which serves as a perfect template for the epitaxial growth of single-oriented NiO and β-Ga2O3 by creating a constant gradient from CeO2 (2.7 Å along ➎001➉) to NiO (2.9 Å along ➎110➉), and eventually to β-Ga2O3 (3.04 Å along ➎010➉). The Hastelloy substrates endow photodetectors with good deformability and mechanical robustness. Moreover, owing to the type-II band alignment of β-Ga2O3/NiO heterostructures, the photodetectors have a good photocurrent at zero bias under 284 nm of light illumination. In addition, the photocurrent is significantly higher than when using an analogous heterostructure (as described in some previous reports), because the β-Ga2O3 and NiO thin films are crystalized along a single orientation with fewer defects.
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Ti3C2Tx MXene van der Waals gate contact for GaN high electron mobility transistors(Advanced Materials, Wiley, 2023-03-20) [Article]Gate controllability is a key factor that determines the performance of GaN high electron mobility transistors (HEMTs). However, at traditional metal-GaN interface, direct chemical interaction between metal and GaN can result in fixed charges and traps, which can significantly deteriorate the gate controllability. In this study, Ti3C2Tx MXene films were integrated into GaN HEMTs as the gate contact, wherein van der Waals heterojunctions were formed between MXene films and GaN without direct chemical bonding. The GaN HEMTs with enhanced gate controllability exhibited an extremely low off-state current (IOFF) of 10−7 mA/mm, a record high ION/IOFF current ratio of ∼1013 (which is six orders of magnitude higher than conventional Ni/Au contact), a high off-state drain breakdown voltage of 1085 V, and a near-ideal subthreshold swing of 61 mV/dec. This work shows the great potential of MXene films as gate electrodes in wide-bandgap semiconductor devices.
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Variability in Resistive Memories(Advanced Intelligent Systems, Wiley, 2023-03-14) [Article]Resistive memories are outstanding electron devices that have displayed a large potential in a plethora of applications such as nonvolatile data storage, neuromorphic computing, hardware cryptography, etc. Their fabrication control and performance have been notably improved in the last few years to cope with the requirements of massive industrial production. However, the most important hurdle to progress in their development is the so-called cycle-to-cycle variability, which is inherently rooted in the resistive switching mechanism behind the operational principle of these devices. In order to achieve the whole picture, variability must be assessed from different viewpoints going from the experimental characterization to the adequation of modeling and simulation techniques. Herein, special emphasis is put on the modeling part because the accurate representation of the phenomenon is critical for circuit designers. In this respect, a number of approaches are used to the date: stochastic, behavioral, mesoscopic..., each of them covering particular aspects of the electron and ion transport mechanisms occurring within the switching material. These subjects are dealt with in this review, with the aim of presenting the most recent advancements in the treatment of variability in resistive memories.
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Nucleation Stage for the Oriented Growth of Tantalum Sulfide Monolayers on Au(111)(The Journal of Physical Chemistry C, American Chemical Society (ACS), 2023-03-13) [Article]We study the nucleation stage in the epitaxial growth of monolayer TaS2 as a model system for monolayer transition-metal sulfides. The growth was done under ultrahigh-vacuum conditions with Au(111) as a substrate on which the metal atoms are evaporated, and the sulfur is provided from a background of H2S. Using scanning tunneling microscopy, we find atomic-scale protrusions with a well-defined triangular shape that act as nuclei for the further growth of extended tantalum sulfide monolayers. We identify these protrusions as TaS3 using density functional theory. We propose that their unique orientation is the cause of the well-defined orientation of a complete TaS2 layer found under favorable growth conditions.
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Copper Organometallic Iodide Arrays for Efficient X-ray Imaging Scintillators(ACS Central Science, American Chemical Society (ACS), 2023-03-10) [Article]Lead-free organic metal halide scintillators with low-dimensional electronic structures have demonstrated great potential in X-ray detection and imaging due to their excellent optoelectronic properties. Herein, the zero-dimensional organic copper halide (18-crown-6)2Na2(H2O)3Cu4I6 (CNCI) which exhibits negligible self-absorption and near-unity green-light emission was successfully deployed into X-ray imaging scintillators with outstanding X-ray sensitivity and imaging resolution. In particular, we fabricated a CNCI/polymer composite scintillator with an ultrahigh light yield of ∼109,000 photons/MeV, representing one of the highest values reported so far for scintillation materials. In addition, an ultralow detection limit of 59.4 nGy/s was achieved, which is approximately 92 times lower than the dosage for a standard medical examination. Moreover, the spatial imaging resolution of the CNCI scintillator was further improved by using a silicon template due to the wave-guiding of light through CNCI-filled pores. The pixelated CNCI-silicon array scintillation screen displays an impressive spatial resolution of 24.8 line pairs per millimeter (lp/mm) compared to the resolution of 16.3 lp/mm for CNCI-polymer film screens, representing the highest resolutions reported so far for organometallic-based X-ray imaging screens. This design represents a new approach to fabricating high-performance X-ray imaging scintillators based on organic metal halides for applications in medical radiography and security screening.
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Nonvolatile Magnetoelectric Switching of Magnetic Tunnel Junctions with Dipole Interaction(Advanced Functional Materials, Wiley, 2023-03-10) [Article]The magnetoelectric effect is technologically appealing because of its ability to manipulate magnetism using an electric field rather than magnetic field or current, thus providing a promising solution for the development of energy-efficient spintronics. Although 180° magnetization switching is vital to spintronic devices, the achievement of 180° magnetization switching via magnetoelectric coupling is still a fundamental challenge. Herein, voltage-driven full resistance switching of a magnetic tunnel junction (MTJ) with dipole interaction on a ferroelectric substrate through switchable parallel/antiparallel magnetization alignment is demonstrated. Parallel magnetization alignment along the y direction is obtained under a bias magnetic field. By rotating the magnetic easy axis via strain-mediated magnetoelectric coupling, the parallel magnetizations in the MTJ reorient to the x axis with opposite paths because of dipole interaction, thus resulting in antiparallel alignment. Moreover, this voltage switching of MTJs is nonvolatile owing to variations in dipole interaction and can be well understood via phase field simulations. The results provide an avenue to realize electrical switching of MTJs and are significant for exploring energy-efficient spintronic devices.
