Quantum Hall Effect in Electron-Doped Black Phosphorus Field-Effect Transistors.

The advent of black phosphorus field-effect transistors (FETs) has brought new possibilities in the study of two-dimensional (2D) electron systems. In a black phosphorus FET, the gate induces highly anisotropic 2D electron and hole gases. Although the 2D hole gas in black phosphorus has reached high carrier mobilities that led to the observation of the integer quantum Hall effect, the improvement in the sample quality of the 2D electron gas (2DEG) has however been only moderate; quantum Hall effect remained elusive.

Discerning Black Phosphorus Crystal Orientation and Anisotropy by Polarized Reflectance Measurement.

Strong in-plane anisotropy of atomic layer and thin-film black phosphorus (P) offers new device perspectives and stimulates increasing interest and explorations, where precisely determining the black P crystal orientation and anisotropic axes is a necessity. Here, we demonstrate that the crystal orientation and intrinsic in-plane optical anisotropy of black P crystals in a broad thickness range (from ∼5 to ∼300 nm) can be directly and precisely determined, by polarized reflectance measurement alone, in visible range. Combining experiments with modeling of optical anisotropy and multilayer interference effects, we elucidate the underlying principles and validate these measurements.

Direct observation of the layer-dependent electronic structure in phosphorene.

Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new two-dimensional (2D) material that holds promise for electronic and photonic technologies. Here we experimentally demonstrate that the electronic structure of few-layer phosphorene varies significantly with the number of layers, in good agreement with theoretical predictions. The interband optical transitions cover a wide, technologically important spectral range from the visible to the mid-infrared.

Quantum Hall effect in black phosphorus two-dimensional electron system.

The development of new, high-quality functional materials has been at the forefront of condensed-matter research. The recent advent of two-dimensional black phosphorus has greatly enriched the materials base of two-dimensional electron systems (2DESs). Here, we report the observation of the integer quantum Hall effect in a high-quality black phosphorus 2DES.

Quantum oscillations in a two-dimensional electron gas in black phosphorus thin films.

For decades, two-dimensional electron gases (2DEG) have allowed important experimental discoveries and conceptual developments in condensed-matter physics. When combined with the unique electronic properties of two-dimensional crystals, they allow rich physical phenomena to be probed at the quantum level. Here, we create a 2DEG in black phosphorus--a recently added member of the two-dimensional atomic crystal family--using a gate electric field.

Black phosphorus field-effect transistors.

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres.