Second-Order Raman Scattering in Exfoliated Black Phosphorus

Abstract

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D, D, D and D, in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the A and A modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D' and D modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these D modes to disorder helps explaining several discrepancies found in the literature.

Strain engineering of the lattice vibration modes in monolayer black phosphorus

“…For a monolayer BP, there are only three vibration modes in the backscattering configuration. Explicitly, the cross‐plane mode stems from the opposing vibrations of the top and bottom phosphorus atoms with respect to each other, the B 2 g mode describes the bond movement along the zigzag direction, and the mode has a dominant component along the in‐plane armchair direction . For simplicity but without loss of generality, we consider the , B 2 g and modes, respectively, move along the x , y and z directions, as shown in Fig.…”

Electrochemical transformation of black phosphorous to phosphorene quantum dots: effect of nitrogen doping

“…These modes evolve as red shifted peaks with decrease in sample thickness. Moreover, earlier studies on phonon dispersion and momentum histogram of BP suggest that the origin of red-shifted characteristics relative to the central peaks is from contributions in the zigzag direction, while the blue shifted characteristics evolve from the armchair direction [15]. This observation is essentially due to the in-plane anisotropy of BP.…”

“…For example, an electron-phonon interaction driven by an impurity, a localized defect, or an edge can generate phonon-defect modes known as D modes. Thanks to these D modes which are helpful in interpreting carrier mobility, level of doping, defects and phonon dispersion [15]. Accordingly, two such phonon-defect modes D 1 and D 2 are identified in the A 1 g and A 2 g regions, respectively for NPQDs.…”

Improved Hole Injection into Perovskite Light‐Emitting Diodes Using A Black Phosphorus Interlayer

“…Unlike hydrophilic filters, PTFE is not wetted by air humidity. [20] Pristine films produced with our method exhibited A g 1 /A g 2 ratio of 0.86, revealing very limited amount of oxygen degradation. Afterwards, the film was transferred onto the substrate by mechanical press.…”