High-k 2D Sb2O3 Made Using a Substrate-Independent and Low-Temperature Liquid-Metal-Based Process.

High dielectric constant (high-k) ultrathin films are required as insulating gate materials. The well-known high-k dielectrics, including HfO2, ZrO2, and SrTiO3, feature three-dimensional lattice structures and are thus not easily obtained in the form of distinct ultrathin sheets. Therefore, their deposition as ultrathin layers still imposes challenges for electronic industries. Consequently, new high-k nanomaterials with k in the range of 40 to 100 and a band gap exceeding 4 eV are highly sought after. Antimony oxide nanosheets appear as a potential candidate that could fulfill these characteristics. Here, we report on the stoichiometric cubic polymorph of 2D antimony oxide (Sb2O3) as an ideal high-k dielectric sheet that can be synthesized via a low-temperature, substrate-independent, and silicon-industry-compatible liquid metal synthesis technique. A bismuth-antimony alloy was produced during the growth process. Preferential oxidation caused the surface of the melt to be dominated by α-Sb2O3. This ultrathin α-Sb2O3 was then deposited onto desired surfaces via a liquid metal print transfer. A tunable sheet thickness between ∼1.5 and ∼3 nm was achieved, while the lateral dimensions were within the millimeter range. The obtained α-Sb2O3 exhibited high crystallinity and a wide band gap of ∼4.4 eV. The relative permittivity assessment revealed a maximum k of 84, while a breakdown electric field of ∼10 MV/cm was observed. The isolated 2D α-Sb2O3 nanosheets were utilized in top-gated field-effect transistors that featured low leakage currents, highlighting that the obtained material is a promising gate oxide for conventional and van der Waals heterostructure-based electronics.

Printable Single-Unit-Cell-Thick Transparent Zinc-Doped Indium Oxides with Efficient Electron Transport Properties.

Ultrathin transparent conductive oxides (TCOs) are emerging candidates for next-generation transparent electronics. Indium oxide (In2O3) incorporated with post-transition-metal ions (e.g., Sn) has been widely studied due to their excellent optical transparency and electrical conductivity. However, their electron transport properties are deteriorated at the ultrathin two-dimensional (2D) morphology compared to that of intrinsic In2O3. Here, we explore the domain of transition-metal dopants in ultrathin In2O3 with the thicknesses down to the single-unit-cell limit, which is realized in a large area using a low-temperature liquid metal printing technique. Zn dopant is selected as a representative to incorporate into the In2O3 rhombohedral crystal framework, which results in the gradual transition of the host to quasimetallic. While the optical transmittance is maintained above 98%, an electron field-effect mobility of up to 87 cm2 V-1 s-1 and a considerable sub-kΩ-1 cm-1 ranged electrical conductivity are achieved when the Zn doping level is optimized, which are in a combination significantly improved compared to those of reported ultrathin TCOs. This work presents various opportunities for developing high-performance flexible transparent electronics based on emerging ultrathin TCO candidates.

Nanoscale Probing of Cholesterol-Rich Domains in Single Bilayer Dimyristoyl-Phosphocholine Membranes Using Near-Field Spectroscopic Imaging.

Cholesterol is believed to induce the formation of membrane domains, "rafts", which are implicated in a range of natural and pathologic membrane processes. Therefore, it is important to understand the role that cholesterol plays in the formation of these structures. Here, we use label-free spectroscopic imaging to investigate cholesterol fractioning in supported bilayer membranes at nanoscale. Scattering-type scanning near-field optical microscopy (s-SNOM) was used to visualize the formation of cholesterol-induced domains in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes. Our results revealed the coexistence of phase separated domains in DMPC lipids with 10 mol % cholesterol content, whereas a mostly homogeneous bilayer was found at low (5 mol %) and high (15 mol %) cholesterol content. Near-field nano-FTIR spectroscopy was used to identify the cholesterol-rich domains based on their qualitative chemical compositions. It was determined that cholesterol binds to phosphodiester and alkyl glycerol ester moieties, likely via hydrogen bonding of the alcohol to either of the ester oxygens. The results also confirm the existence of an ideal cholesterol-lipid mixture ratio (∼15:85) with a geometrically defined packing. At lower cholesterol content there is phase separation between liquid ordered and almost neat DMPC domains. Thus, the liquid ordered phase exists at an energy minimum at a given lipid-cholesterol ratio.

Atomically thin TiO2 nanosheets synthesized using liquid metal chemistry.

The library of true two-dimensional materials is limited since many transition metal compounds are not stratified and can thus not be easily isolated as nanosheets. Here, micron-sized ultrathin rutile TiO2 nanosheets featuring uniform thickness (2 ± 0.5 nm) with dielectric constant (ε⊥ = 24) have been synthesized via a liquid metal synthesis strategy.

Exciton-Driven Chemical Sensors Based on Excitation-Dependent Photoluminescent Two-Dimensional SnS.

Excitation wavelength-dependent photoluminescence (PL) in two-dimensional (2D) transition-metal chalcogenides enables a strong excitonic interaction for high-performance chemical and biological sensing applications. In this work, we explore the possible candidates in the domain of post-transition-metal chalcogenides. Few-layered 2D p-type tin monosulfide (SnS) nanoflakes with submicrometer lateral dimensions are synthesized from the liquid phase exfoliation of bulk crystals. Excitation wavelength-dependent PL is found, and the excitonic radiative lifetime is more than one order enhanced compared to that of the bulk counterpart because of the quantum confinement effect. Paramagnetic NO2 gas is selected as a representative to investigate the exciton-driven chemical-sensing properties of 2D SnS. Physisorption of NO2 results in the formation of dipoles on the surface of 2D SnS, causing the redistribution of photoexcited charges in the body and therefore modifying PL properties. For practical sensing applications, 2D SnS is integrated into a resistive transducing platform. Under light irradiation, the sensor exhibits excellent sensitivity and selectivity to NO2 at a relatively low operating temperature of 60 °C. The limit of detection is 17 parts per billion (ppb), which is significantly improved over other previously reported 2D p-type semiconductor-based NO2 sensors.

Wafer-Sized Ultrathin Gallium and Indium Nitride Nanosheets through the Ammonolysis of Liquid Metal Derived Oxides.

We report the synthesis of centimeter sized ultrathin GaN and InN. The synthesis relies on the ammonolysis of liquid metal derived two-dimensional (2D) oxide sheets that were squeeze-transferred onto desired substrates. Wurtzite GaN nanosheets featured typical thicknesses of 1.3 nm, an optical bandgap of 3.5 eV and a carrier mobility of 21.5 cm2 V-1 s-1, while the InN featured a thickness of 2.0 nm. The deposited nanosheets were highly crystalline, grew along the (001) direction and featured a thickness of only three unit cells. The method provides a scalable approach for the integration of 2D morphologies of industrially important semiconductors into emerging electronics and optical devices.

Printing two-dimensional gallium phosphate out of liquid metal.

Two-dimensional piezotronics will benefit from the emergence of new crystals featuring high piezoelectric coefficients. Gallium phosphate (GaPO4) is an archetypal piezoelectric material, which does not naturally crystallise in a stratified structure and hence cannot be exfoliated using conventional methods. Here, we report a low-temperature liquid metal-based two-dimensional printing and synthesis strategy to achieve this goal. We exfoliate and surface print the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide bandgap two-dimensional (2D) GaPO4 nanosheets of unit cell thickness, while featuring lateral dimensions reaching centimetres. The unit cell thick nanosheets present a large effective out-of-plane piezoelectric coefficient of 7.5 ± 0.8 pm V-1. The developed printing process is also suitable for the synthesis of free standing GaPO4 nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials.

Bi2O3 monolayers from elemental liquid bismuth.

Atomically thin, semiconducting transition and post transition metal oxides are emerging as a promising category of materials for high-performance oxide optoelectronic applications. However, the wafer-scale synthesis of crystalline atomically thin samples has been a challenge, particularly for oxides that do not present layered crystal structures. Herein we use a facile, scalable method to synthesise ultrathin bismuth oxide nanosheets using a liquid metal facilitated synthesis approach. Monolayers of α-Bi2O3 featuring sub-nanometre thickness, high crystallinity and large lateral dimensions could be isolated from the liquid bismuth surface. The nanosheets were found to be n-type semiconductors with a direct band gap of ∼3.5 eV and were suited for developing ultra violet (UV) photodetectors. The developed devices featured a high responsivity of ∼400 AW-1 when illuminated with 365 nm UV light and fast response times of ∼70 µs. The developed methods and obtained nanosheets can likely be developed further towards the synthesis of other bismuth based atomically thin chalcogenides that hold promise for electronic, optical and catalytic applications.