Effects of plasma-treatment on the electrical and optoelectronic properties of layered black phosphorus (original) (raw)
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Isolation and characterization of few-layer black phosphorus
2014
Isolation and characterization of mechanically exfoliated black phosphorus flakes with a thickness down to two single-layers is presented. A modification of the mechanical exfoliation method, which provides higher yield of atomically thin flakes than conventional mechanical exfoliation, has been developed. We present general guidelines to determine the number of layers using optical microscopy, Raman spectroscopy and transmission electron microscopy (TEM) in a fast and reliable way. Moreover, we demonstrate that the exfoliated flakes are highly crystalline and that they are stable even in free-standing form through Raman spectroscopy and TEM measurements. A strong thickness dependence of the band structure is found by density functional theory (DFT) calculations. The exciton binding energy, within an effective mass approximation, is also calcu- lated for different number of layers. Our computational results for the optical gap are consistent with preliminary photoluminescence results on thin flakes. Finally, we study the environmental stability of black phosphorus flakes finding that the flakes are very hydrophilic and that long term exposure to air moisture etches black phosphorus away. Nonetheless, we demonstrate that the aging of the flakes is slow enough to allow fabrication of field-effect transistors with strong ambipolar behavior. DFT calculations also give us insight into the water-induced changes of the structural and electronic properties of black phosphorus.
arXiv (Cornell University), 2014
Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of P atoms stacked together by weak van der Waals forces can be exfoliated down to the stratophosphane monolayer (also called phosphorene) using procedures similar to those used for graphene. Properties of this 2D material are however challenging to probe due to a fast and ubiquitous degradation upon exposure to ambient conditions. Herein, we investigate the crystal degradation using in-situ Raman and transmission electron spectroscopies and highlight a process involving a photo-induced oxidation reaction with adsorbed oxygen in water. The experimental conditions to prepare and preserve mono-, bi-and multilayers of stratophosphane in their pristine states were determined. Study on these 2D layers provides new insights on the effect of confinement on the chemical reactivity and the vibrational modes of black phosphorus.
Nature communications, 2014
Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600 cm 2 V À 1 s À 1 for holes along the light (x) and heavy (y) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 mm. Field-effect transistors using 5 nm BP along x direction exhibit an on-off current ratio exceeding 10 5 , a field-effect mobility of 205 cm 2 V À 1 s À 1 , and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable.
Ultrathin black phosphorus, or phosphorene, is the second known elementary two-dimensional material that can be exfoliated from a bulk van der Waals crystal. Unlike graphene it is a semiconductor with a sizeable band gap and its excellent electronic properties make it attractive for applications in transistor, logic, and optoelectronic devices. However, it is also the first widely investigated two dimensional electronic material to undergo degradation upon exposure to ambient air. Therefore a passivation method is required to study the intrinsic material properties, understand how oxidation affects the physical transport properties and to enable future application of phosphorene. Here we demonstrate that atomically thin graphene and hexagonal boron nitride crystals can be used for passivation of ultrathin black phosphorus. We report that few-layer pristine black phosphorus channels passivated in an inert gas environment, without any prior exposure to air, exhibit greatly improved n-type charge transport resulting in symmetric electron and hole trans-conductance characteristics. We attribute these results to the formation of oxygen acceptor states in air-exposed samples which drastically perturb the band structure in comparison to the pristine passivated black phosphorus.
Triangular Black Phosphorus Atomic Layers by Liquid Exfoliation
Scientific Reports, 2016
Few-layer black phosphorus (BP) is the most promising material among the two-dimensional materials due to its layered structure and the excellent semiconductor properties. Currently, thin BP atomic layers are obtained mostly by mechanical exfoliation of bulk BP, which limits applications in thin-film based electronics due to a scaling process. Here we report highly crystalline few-layer black phosphorus thin films produced by liquid exfoliation. We demonstrate that the liquid-exfoliated BP forms a triangular crystalline structure on SiO 2 /Si (001) and amorphous carbon. The highly crystalline BP layers are faceted with a preferred orientation of the (010) plane on the sharp edge, which is an energetically most favorable facet according to the density functional theory calculations. Our results can be useful in understanding the triangular BP structure for large-area applications in electronic devices using twodimensional materials. The sensitivity and selectivity of liquid-exfoliated BP to gas vapor demonstrate great potential for practical applications as sensors.
Phosphorus oxide gate dielectric for black phosphorus field effect transistors
Applied Physics Letters
The environmental stability of the layered semiconductor black phosphorus (bP) remains a challenge. Passivation of the bP surface with phosphorus oxide, PO x , grown by a reactive ion etch with oxygen plasma is known to improve photoluminescence efficiency of exfoliated bP flakes. We apply phosphorus oxide passivation in the fabrication of bP field effect transistors using a gate stack consisting of a PO x layer grown by reactive ion etching followed by atomic layer deposition of Al 2 O 3. We observe room temperature top-gate mobilities of 115 cm 2 V À1 s À1 in ambient conditions, which we attribute to the low defect density of the bP/PO x interface. V
Thin layers of black phosphorus have recently raised interest for their two-dimensional semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of P atoms stacked together by weak van der Waals forces can be exfoliated down to the stratophosphane monolayer (also called phosphorene) using procedures similar to those used for graphene. Properties of this new material are however challenging to probe due to a fast and ubiquitous degradation upon exposure to ambient conditions. Herein, we investigate the crystal degradation using in-situ Raman and transmission electron spectroscopies and highlight a process involving a photo-induced oxidation reaction with adsorbed oxygen in water. The experimental conditions to prepare and preserve stratophosphane mono-, bi- and multi-layers in their pristine states were determined. Study of these pristine layers provides new insights on the effect of confinement on the chemical reactivity and the vi...
Plasma-Assisted Synthesis of High-Mobility Atomically Layered Violet Phosphorus
ACS applied materials & interfaces, 2015
Two-dimensional layered materials such as graphene, transition metal dichalcogenides, and black phosphorus have demonstrated outstanding properties due to electron confinement as the thickness is reduced to atomic scale. Among the phosphorus allotropes, black phosphorus, and violet phosphorus possess layer structure with the potential to be scaled down to atomically thin film. For the first time, the plasma-assisted synthesis of atomically layered violet phosphorus has been achieved. Material characterization supports the formation of violet phosphorus/InN over InP substrate where the layer structure of violet phosphorus is clearly observed. The identification of the crystal structure and lattice constant ratifies the formation of violet phosphorus indeed. The critical concept of this synthesis method is the selective reaction induced by different variations of Gibbs free energy (ΔG) of reactions. Besides, the Hall mobility of the violet phosphorus on the InP substrate greatly incre...