Engineering Top Gate Stack for Wafer-Scale Integrated Circuit Fabrication Based on Two-Dimensional Semiconductors.

In recent years, two-dimensional (2D) semiconductors have attracted considerable attention from both academic and industrial communities. Recent research has begun transforming from constructing basic field-effect transistors (FETs) into designing functional circuits. However, device processing remains a bottleneck in circuit-level integration. In this work, a non-destructive doping strategy is proposed to modulate precisely the threshold voltage (VTH) of MoS2-FETs in a wafer scale. By inserting an Al interlayer with a varied thickness between the high-k dielectric and the Au top gate (TG), the doping could be controlled. The full oxidation of the Al interlayer generates a surplus of oxygen vacancy (Vo) in the high-k dielectric layer, which further leads to stable electron doping. The proposed strategy is then used to optimize an inverter circuit by matching the electrical properties of the load and driver transistors. Furthermore, the doping strategy is used to fabricate digital logic blocks with desired logic functions, which indicates its potential to fabricate fully integrated multistage logic circuits based on wafer-scale 2D semiconductors.

Strain engineering and lattice vibration manipulation of atomically thin TaS2 films.

Beside the extraordinary structural, mechanical and physical properties of two-dimensional (2D) materials, the capability to tune properties via strain engineering has shown great potential for nano-electromechanical systems. External strain, in a controlled manner, can manipulate the optical and electronic properties of the 2D materials. We observed the lattice vibration modulation in strained mono- and few-layer tantalum sulfide (TaS2). Two Raman modes, E1g and E1 2g, exhibit sensitive strain dependence, with the frequency of the former intensity increasing and the latter decreasing under a compressive strain. The opposite direction of the intensity shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to strain-induced competition between the electron-phonon interlayer coupling and possible stacking-induced changes of the intralayer transport. Our results enrich the understanding of the lattice vibration of TaS2 and point to strain engineering as a powerful tool for tuning the electron-phonon coupling of 2D materials.

Mechanical-Induced Polarization Switching in Relaxor Ferroelectric Single Crystals.

Control of coupling between electric and elastic orders in ferroelectric bulks is vital to understand their nature and enrich the multifunctionality of polarization manipulation applied in domain-based electronic devices such as ferroelectric memories and data storage ones. Herein, taking (1 - x%)Pb(Mg1/3Nb2/3)O3-x%PbTiO3 (PMN-x%PT, x = 32, 40) as the prototype, we demonstrate the less-explored mechanical switching in relaxor ferroelectric single crystals using scanning probe microscopy. Low mechanical forces can induce metastable and electrically erasable polarization reversal clearly from electrical-created bipolar domains around the 180° domain wall in monoclinic PMN-32%PT and inside the c+ domain in tetragonal PMN-40%PT. The mechanical switching evolutions show force/time dependence and time-force equivalence. The time-dependent mechanical switching behavior stems from the participation and contribution of polar nanoregions. Flexoelectricity and bulk Vegard strain effect can account for the mechanical switching but notably, the former in the two has very different origins. These investigations exhibit the possibility of mechanical switching as a tool to manipulate polarization states in ferroelectric bulks, and provide the potential of these crystals as substrates in mechanical polarization control of future thin-film devices.

Remarkably Enhanced Negative Electrocaloric Effect in PbZrO3 Thin Film by Interface Engineering.

The electrocaloric effect in ferroelectric materials has drawn much attention due to its potential applications in integrated circuit cooling and novel cooling devices. In contrast to the widely researched positive electrocaloric effect, the negative electrocaloric effect has received much less attention due to the lack of any effective methods for significant enhancement. In this work, we fabricated PbZrO3 thin film on a Pt/Si substrate by the sol-gel method. By controlling the interface conditions between the thin film and substrate, we induced defects into the interface and stabilized a transient ferroelectric phase in the PbZrO3 thin film. The emergence of the transient ferroelectric phase postpones the antiferroelectric-ferroelectric phase transition. As a result, a negative electrocaloric effect up to -18.5 K is estimated near room temperature, the highest one ever reported in this temperature range. This result suggests a new strategy to enhance the negative electrocaloric effect and may benefit the application of PbZrO3 thin films in cooling devices.

Enhanced photocatalytic activity of perovskite NaNbO3 by oxygen vacancy engineering.

NaNbO3 with oxygen vacancies has been successfully synthesized through a well controllable solid-state reaction, whose photocatalytic performances have been prominently enhanced by almost 2.4 times compared with just annealed NaNbO3 (the control sample). When oxygen vacancies were introduced into the perovskite, the color of NaNbO3 turned black and the band gap was decreased, resulting in its remarkable absorption under visible light, and its higher symmetry also favors the electron transfer. More importantly, oxygen vacancies lead to larger specific surface area and higher charge density, which play non-negligible roles in improving the visible-light-activities. These encouraging findings prove that oxygen vacancy engineering is a feasible and general strategy to improve the photocatalytic performances of perovskite oxides, which will promote many related applications.

Epitaxial Lift-Off of Centimeter-Scaled Spinel Ferrite Oxide Thin Films for Flexible Electronics.

Mechanical flexibility of electronic devices has attracted much attention from research due to the great demand in practical applications and rich commercial value. Integration of functional oxide materials in flexible polymer materials has proven an effective way to achieve flexibility of functional electronic devices. However, the chemical and mechanical incompatibilities at the interfaces of dissimilar materials make it still a big challenge to synthesize high-quality single-crystalline oxide thin film directly on flexible polymer substrates. This study reports an improved method that is employed to successfully transfer a centimeter-scaled single-crystalline LiFe5 O8 thin film on polyimide substrate. Structural characterizations show that the transferred films have essentially no difference in comparison with the as-grown films with respect to the microstructure. In particular, the transferred LiFe5 O8 films exhibit excellent magnetic properties under various mechanical bending statuses and show excellent fatigue properties during the bending cycle tests. These results demonstrate that the improved transfer method provides an effective way to compose single-crystalline functional oxide thin films onto flexible substrates for applications in flexible and wearable electronics.

Collagen intrafibrillar mineralization as a result of the balance between osmotic equilibrium and electroneutrality.

Mineralization of fibrillar collagen with biomimetic process-directing agents has enabled scientists to gain insight into the potential mechanisms involved in intrafibrillar mineralization. Here, by using polycation- and polyanion-directed intrafibrillar mineralization, we challenge the popular paradigm that electrostatic attraction is solely responsible for polyelectrolyte-directed intrafibrillar mineralization. As there is no difference when a polycationic or a polyanionic electrolyte is used to direct collagen mineralization, we argue that additional types of long-range non-electrostatic interaction are responsible for intrafibrillar mineralization. Molecular dynamics simulations of collagen structures in the presence of extrafibrillar polyelectrolytes show that the outward movement of ions and intrafibrillar water through the collagen surface occurs irrespective of the charges of polyelectrolytes, resulting in the experimentally verifiable contraction of the collagen structures. The need to balance electroneutrality and osmotic equilibrium simultaneously to establish Gibbs-Donnan equilibrium in a polyelectrolyte-directed mineralization system establishes a new model for collagen intrafibrillar mineralization that supplements existing collagen mineralization mechanisms.

Novel lead-free ferroelectric film by ultra-small Ba0.8Sr0.2TiO3 nanocubes assembled for a large electrocaloric effect.

A giant electrocaloric effect (ECE) can be achieved in ferroelectric thin films, which demonstrates the applications of thin films in alternative cooling. However, electrocaloric thin films fabricated by conventional techniques, such as the pulsed laser deposition or sol-gel methods, may be limited by high costs, low yield and their dependence on substrates. In this study, we present a new bottom-up strategy to construct electrocaloric Ba0.8Sr0.2TiO3 thin films by assembling precisely designed building blocks of ferroelectric nanocubes, which is supported by detailed structural characterization. Moreover, it is found that our assembled Ba0.8Sr0.2TiO3 films differ remarkably from both individual Ba0.8Sr0.2TiO3 NPs and bulk Ba0.8Sr0.2TiO3 ceramics in terms of new collective ferroelectric properties, including superior and diffused permittivity constants and polarization-electric field loops. Benefiting from these unique ferroelectric properties, a giant ECE (9.1 K) over a broad temperature range (20 °C to 60 °C) is achieved, which is very large in the lead-free oxide film. Clearly, this bottom-up strategy provides a promising pathway for developing high electrocaloric effect devices.