Direct observation of the layer-dependent electronic structure in phosphorene

Abstract

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...

Optical third harmonic generation in black phosphorus

“…The optical conductivity of bulk is in agreement with reports on extinction spectra [52] and with the dielectric function computed from electron energy loss spectroscopy (EELS) data [53]. Results for monolayer are consistent with previous calculations in the single-particle approximation [14,20,21] but show limitations of this approximation by not accounting for excitonic resonances present in monolayer bP [17,52,54].…”

“…This parametrization leads to energy dispersion consistent with ab initio results [14,17,34] for monolayer but overestimates the bulk gap. For bulk, the gap [17] can be recovered by rescaling the coupling between different layers with a factor of ∼0.54, or conversely by stretching the layer separation by ∼9%. The latter was used to generate all results computed in this work.…”

Exploring the Formation of Black Phosphorus Intercalation Compounds with Alkali Metals

“…The individual layers of BP exhibit a honeycomb structure differing from that of graphene in that it has a marked puckering of the sp 3 P atoms with an AB stacking structure driven by van der Waals interactions (Scheme 1). 3, 4 BP has attracted enormous attention owing to its high p‐type charge carrier mobility up to around 6000 cm 2  V −1  s −1 and its tunable direct band gap ranging from about 0.3 eV for bulk to approximately 2 eV for monolayers 2, 9, 10, 11, 12, 13, 14, 15, 16. It has recently been predicted that the electronic properties of BP can be modulated by strain or electric field 17, 18.…”

Midinfrared Electro-optic Modulation in Few-Layer Black Phosphorus

“…Strong field-effect tuning of its bandgap has been observed in BP, and its infrared optical response has been measured [36][37][38] . It's anisotropic band structure and interband coupling also imbued it with peak-like subband absorption features, which is advantageous for low -3 -voltage optical modulation [39][40][41] . Here we report the broadband measurement of the electrical modulation of the optical absorption in multilayer BP, corroborated with the theoretical calculation of the optical conductivity obtained from Kubo formula based on the tight-binding description 39,40 .…”