From a fundamental science perspective, black phosphorus (BP) is a canonical example of a material that possesses fascinating surface and electronic properties. It has extraordinary in-plane anisotropic electrical, optical, and vibrational states, as well as a tunable band gap. However, instability of the surface due to chemical degradation in ambient conditions remains a major impediment to its prospective applications. Early studies were limited by the degradation of black phosphorous surfaces in air. Recently, several robust strategies have been developed to mitigate these issues, and these novel developments can potentially allow researchers to exploit the extraordinary properties of this material and devices made out of it. Here, the fundamental chemistry of BP degradation and the tremendous progress made to address this issue are extensively reviewed. Device performances of encapsulated BP are also compared with nonencapsulated BP. In addition, BP possesses sensitive anisotropic photophysical surface properties such as excitons, surface plasmons/phonons, and topologically protected and Dirac semi-metallic surface states. Ambient degradation as well as any passivation method used to protect the surface could affect the intrinsic surface properties of BP. These properties and the extent of their modifications by both the degradation and passivation are reviewed.
2D materials are considered promising candidates for developing next‐generation high‐performance energy efficient electronic, optoelectronic, and valley‐tronic devices. Though metal oxides are widely used in the fabrication of many advanced devices, very little work has been reported on their properties in 2D limit. This article reports the discovery of a new 2D materials system, 2D tin monoxide (SnO). Layer by layer growth of SnO on sapphire and SiO2 substrates is demonstrated using a pulsed laser deposition method. The number of SnO layers is controlled by controlling the number of laser shots during the deposition process. Raman spectroscopic and X‐ray photoelectron spectroscopic analysis confirms the formation of phase pure SnO layers. Field effect transistors (FETs) using few layer SnO channels grown on SiO2 substrates are successfully fabricated. These FETs show typical p‐channel conduction with field effect mobility ranging from 0.05 to 1.9 cm2 V−1 s−1. Field effect mobility varies with the number of SnO layers and decreases on either sides of the optimum layer numbers (12), which is explained based on charge screening and interlayer coupling in layered materials.
The authors report the synthesis of CuBO2, a p-type transparent oxide belonging to Cu-delafossite family. High quality thin films of CuBO2 were deposited on c-plane sapphire substrates by pulsed laser deposition technique. Detailed structural, optical, and electrical characterizations on these films were performed. Optical transmission measurement showed the films to be highly transparent in the visible range and having indirect and direct band gaps of 2.2 and 4.5eV, respectively. Room-temperature electrical conductivity of CuBO2 films was 1.65Scm−1 and exhibited semiconductorlike temperature dependence. The Hall and Seebeck coefficients of the film were positive, indicating the p-type nature of the material.
Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio-frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high-frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state-of-the-art values for electrical performance, with electron mobility exceeding 2000 cm V s and sheet carrier density above 1.07 × 10 cm . The influence of strain on the RF performance of flexible GaN high-electron-mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.
Black phosphorus (BP) has recently attracted significant attention due to its exceptional physical properties. Currently, high-quality few-layer and thin-film BP are produced primarily by mechanical exfoliation, limiting their potential in future applications. Here, the synthesis of highly crystalline thin-film BP on 5 mm sapphire substrates by conversion from red to black phosphorus at 700 °C and 1.5 GPa is demonstrated. The synthesized ≈50 nm thick BP thin films are polycrystalline with a crystal domain size ranging from 40 to 70 µm long, as indicated by Raman mapping and infrared extinction spectroscopy. At room temperature, field-effect mobility of the synthesized BP thin film is found to be around 160 cm V s along armchair direction and reaches up to about 200 cm V s at around 90 K. Moreover, red phosphorus (RP) covered by exfoliated hexagonal boron nitride (hBN) before conversion shows atomically sharp hBN/BP interface and perfectly layered BP after the conversion. This demonstration represents a critical step toward the future realization of large scale, high-quality BP devices and circuits.
Two-dimensional (2D) crystals have renewed opportunities in design and assembly of arti cial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high delity. Recent availability of uniform, waferscale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm 2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers.
We report the observation of room temperature ferromagnetism in Cu-doped (5%) ZnO films grown on c-plane sapphire substrates. Films were prepared by pulsed laser deposition technique and were thoroughly characterized using several state-of-the-art characterization techniques. Hall measurements showed that the films are of n-type with a carrier concentration of 3×1017cm−3. Magnetization measurements showed that the films exhibit room temperature ferromagnetism with a saturation magnetization of ∼1.45μB∕Cu atom. When additional carriers were introduced in the films, ferromagnetism was completely vanished. Our results show that the p-type nature of the film is not essential for realizing ferromagnetic characteristics; however, the concentration of n-type carriers should not exceed a critical value.
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