Abstract:Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new twodimensional (2D) material that holds promise for electronic and photonic technology. 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 spectrum range from visible to mid-infrared. In addition, we observe strong photoluminescence in few-layer phosphorene at energies that match well with the absorption edge, indicating they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high electrical mobility, gives it distinct advantages over other twodimensional materials in electronic and opto-electronic applications.Page 3 of 17! ! Atomically thin 2D crystals have emerged as a new class of materials with unique material properties that are potentially important for electronic and photonic technologies [1][2][3][4][5][6][7][8][9][10] . Various 2D crystals have been uncovered, ranging from metallic (and superconducting) NbSe 2 and semimetallic graphene to semiconducting MoS 2 and insulating hexagonal boron nitride (hBN).The energy bandgap, a defining characteristic of an electronic material, varies correspondingly from 0 (in metals and graphene) to 5.8 eV (in hBN) in these 2D crystals. Despite the rich variety currently available, 2D materials with a bandgap in the range from 0.3 eV to 1.5 eV are notably missing 11 . Such a bandgap corresponds to a spectral range from mid-infrared to near-infrared that is important for optoelectronic technologies such as telecommunication and solar energy harvesting. It is therefore desirable to have 2D materials with a bandgap that falls in this range, and in particular, matches that of the technologically important silicon (bandgap = 1.1 eV) and III-V semiconductors like InGaAs, without compromising sample mobility 12 .Monolayer and few-layer phosphorene are predicted to bridge the much needed bandgap range from 0.3 to 2 eV (Refs. 13-17). Inside monolayer phosphorene, each phosphorus atom is covalently bonded with three adjacent phosphorus atoms to form a puckered honeycomb structure 18 . The three near sp 3 bonds together with the lone-pair orbital take up all five valence electrons of phosphorus, so monolayer phosphorene is a semiconductor with a predicted direct optical bandgap of ~ 1.5 eV at the Γ point of the Brillouin zone. The bandgap in few-layer phosphorene can be strongly modified by interlayer interactions, which leads to a bandgap that decreases with phosphorene film thickness, eventually reaching 0.3 eV in the bulk limit.Experimental observations of layer-dependent band structure in phosphorene, on the other hand, have been rather limited. Previously, photoluminescence (PL) spectroscopy has been used to probe the bandgap of monolayer and few-layer phosphorene 8,[19][20][21][22] . Such studies, howeve...

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

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 material base of two-dimensional electron systems.Significant progress has been made to achieve high mobility black phosphorus twodimensional electron gas (2DEG) since the development of the first black phosphorus field-effect transistors (FETs) [1][2][3][4] . Here, we reach a milestone in developing high quality black phosphorus 2DEG -the observation of integer quantum Hall (QH) effect. We achieve high quality by embedding the black phosphorus 2DEG in a van der Waals heterostructure close to a graphite back gate; the graphite gate screens the impurity potential in the 2DEG, and brings the carrier Hall mobility up to 6000. The exceptional mobility enabled us, for the first time, to observe QH effect, and to gain important information on the energetics of the spin-split Landau levels in black phosphorus. Our results set the stage for further study on quantum transport and device application in the ultrahigh mobility regime.Quantum Hall effect, the emergence of quantized Hall resistance in 2DEG sample when subjected to low temperatures and strong magnetic fields, has had a lasting impact in modern condensed matter research. The exact, universal quantization regardless of detailed sample geometry and impurity configuration has enabled the establishment of a metrological resistance standard, and served as the basis for an independent determination of the fine structure constant 5 . Even though the exact quantization of the Hall resistance relies on certain amount of impurities 6 , the observation of QH effect, paradoxically, requires high-purity, low-defect specimens. Because of the stringent requirement on the ). In this work, we achieved high Hall mobility in black phosphorus FETs that is significantly higher than previous record value. This is accomplished by constructing a van der Waals heterostructure with the few-layer black phosphorus sandwiched between two hBN flakes (Fig. 1a,b) and placed on graphite back gate. The top hBN protects the black phosphorus flakes from sample degradation in air. More importantly, the thin bottom hBN (thickness ~ 25 nm) allows the electrons in the graphite to screen the impurity potential at the black phosphorus-hBN interface, where the 2DEG resides. The high mobility enable us, for the first time, to observe the QH effect in black Page 4 of 15 phosphorus 2DEG. Black phosphorus thus joins the selected few materials 5,7,8,11 to become the only 2D atomic crystal apart from graphene 9,10 having requisite material quality to show QH effect.We constructed the van der Waals heterostructure using the dry-transfer technique described in ref. 31. We first cleaved graphite and h-BN flakes onto SiO2/Si wafers, and black phosphorus flake onto poly-propylene carbon (PPC) film. The black phosphorus flake on the PPC film was then used to pick up the h-BN flake on the SiO2/Si wafer. Finally, the black phosphor...

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