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

PQDs and NPQDs were synthesized from BP according to our previously reported single step electrochemical method as schematically represented in figure 1 [10, 12]. Briefly, 50 μl of 2.0 mg ml⁻¹ dispersion of BP (ACS materials) in deaerated ethanol was drop-dried on a 3 mm glassy carbon working electrode (GCE). The electrochemical exfoliation was carried out in a three-electrode cell where the BP coated GCE was the working electrode, Pt wire as the quasi-reference electrode and Pt mesh as the counter electrode. To obtain PQDs, lithium perchlorate (LiClO₄, 0.1 wt %) in Argon-saturated propylene carbonate was used as the electrolyte, while for NPQDs tetraethylammonium tetrafluoroborate (0.1 wt %) in acetonitrile was used. An electric field of strength 5.2 × 10⁸ V m⁻¹ was applied to BP. The product was then separated from the electrolyte by centrifugation. The nitrogen content in NPQDs were analysed using X-ray Photoelectron Spectroscopy which is 3.8 at% [10]. A systematic approach to vary the nitrogen content by controlling the electric field and time was not very successful to yield effective composition modulation with similar lateral sizes or number of layers.

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Figure 2 illustrates a comparison of the transmission electron micrographs (TEM) of electrosynthesized PQDs and NPQDs. The average diameter of PQDs is 8 nm (figure 2(a)) and they are crystalline (figure 2(b)) as evident from the High-Resolution Transmission Electron Micrograph (HR-TEM) showing lattice fringes with a spacing of 0.23 nm which can be assigned to (041) plane of orthorhombic BP as per the refined powder diffraction data of BP [13]. The PQDs also exhibit a fair particle size distribution as represented by the particle size histogram (inset of figure 2(a)). A slightly smaller average size is noticed for NPQDs (figure 2(c)) which is ∼6 nm. NPQDs also exhibit a fair distribution (inset of figure 2(c)) and crystallinity with an ordered lattice fringes corresponding to the orthogonally symmetric structure of BP as observed from the HR-TEM (figure 2(d)). From the aerial view, a P-P distances of 0.16 and 0.23 nm are calculated from x and z directions, respectively. In principle, the inter planar distances calculated from the lattice fringes observed in HR-TEM of a phosphorene lattice should be 0.17 nm and 0.22 nm for x and z direction, respectively. However, we noticed a slight increment along the z direction confirming the atomic reconstruction underwent by phosphorene lattice during transformation to NPQDs.

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Further the crystal structure of PQDs and NPQDs were compared by X-Ray Diffraction (XRD) analysis. Accordingly, figure 3 shows the comparative XRD patterns of PQDs and NPQDs. Many of the diffraction pattern intensities are significantly diminished for both PQDs and NPQDs due to reduced layer thickness and hence van der Waals interactions. For PQDs, a remarkable increase in the in-plane lattice spacing from 0.26 nm (for BP) to 0.28 nm corresponding to (040) facet is observed presumably stemming from the atomic reconstructions in PQDs to maintain stability. On the contrary, for NPQDs, an increase in interlayer distance from 0.53 nm (for BP) to 0.63 nm is noticed corresponding to the shift of reflection from this (020) plane possibly due to the incorporation of functional moieties such as P-NH₂. Moreover a slight lattice contraction is also noticed by the shift of reflection from (040) to a slightly higher 2θ value. This can be attributed the electronegativity effect of nitrogen atoms.

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Effect of nitrogen doping on the crystal structure of phosphorene is further analysed by Raman spectroscopy. Accordingly, figure 4 shows comparative Raman spectra of PQDs and NPQDs obtained using a 514 nm excitation laser. Three characteristic A¹_g, B_2g, and A²_g vibrational modes are observed for BP at wavenumbers of ∼361.8, ∼437.98, and ∼465.19 cm⁻¹ respectively. These can be correspondingly assigned to out-of-plane and in-plane vibration (B_2g, and A²_g) of P atoms respectively. As evident from the spectra, the first order Raman mode A¹_g shows a blue shift of 17 cm⁻¹, while the intensity of B_2g and A²_g diminished tremendously. The morphological transformation affects the vibrations of P atoms apart from the reduced van der Waals interactions. Moreover peak broadening is also observed which can be attributed to the size effects. On the other hand, upon nitrogen doping, A¹_g, B_2g, and A²_g vibrational modes exhibit a red-shift of 9 cm⁻¹, 18 cm⁻¹ and 8.3 cm⁻¹ respectively featuring a phonon softening [14]. It is also important to recall that Raman spectroscopic analysis not only involves one phonon process known as first-order Raman scattering but also higher-order processes. 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 D₁ and D₂ are identified in the A¹_g and A²_g regions, respectively for NPQDs. 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. Therefore NPQDs are more likely to be dominated by zigzag edges while the blue shift observed for PQDs are presumably due to armchair edges. The D peaks also give clues about degradation dynamics. For example, the D₂ mode appears as a result of exposure to light and humidity [15]. All these observations suggest a phonon-defect scattering which is essentially due to the effect of applied electric field and doping. However, an in-depth understanding of origin of D modes can be only obtained from samples deliberately exposed to degradation inducing conditions.

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The AFM topographic images along with their height profiles. PQDs show ∼3–4 layers based on an interlayer distance of 0.53 while lesser number of layers are noticed for NPQDs (∼1–2 layers). This however, does not seem to have any special significance with respect to doping as it could be due to the size differences related to the preparation procedure.

The optical properties of PQDs and NPQDs were probed by UV-Visible and Photoluminescence (PL) spectroscopy and Time-Correlated Single Photon Counting (TCSPS) measurements. Consequently, figure 5(a) shows superimposed UV-Visible spectra of PQDs and NPQDs. Both PQDs and NPQDs exhibit broad UV absorption between 300–400 nm corresponding to optical transition (direct band gap) between the valence band and conduction band which is a characteristic of luminophores. This absorbance is responsible for the blue luminescence of PQDs and NPQDs. A slight red shift is noticed in the absorption of NPQDs. The absorption in the UV spectra is further corroborated with PL measurements. Accordingly figure 5(b) shows the superimposed PL emission spectra of PQDs and NPQDs. As evident from the spectra, three peaks at 406, 429 and 457 nm are exhibited by PQDs (Excitation wavelength = 350 nm). The three emission peaks in the PL spectrum can be assigned to the Highest Occupied Molecular Orbital (HOMO) - Lowest Unoccupied Molecular Orbital (LUMO) electronic transitions among the energy levels of these luminophores (functional groups) on PQDs. The strongest emission is observed at 429 nm corresponding to blue luminescence. A stoke shift of 79 nm is calculated for this transition. This stoke shift is attributed to the non-radiative decay processes occurring at the various energy levels of excited state. Notably, a similar energy difference is noticed between the peaks.

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PL spectrum of NPQDs also exhibits three peaks corresponding to the wavelength of blue region of the electromagnetic spectrum. The strongest PL emission is observed at 424 nm for an excitation wavelength of 360 nm. A shorter Stokes shift is noticed (64 nm) upon doping. More significantly, this spectrum exhibits a higher quantum efficiency of 88.7% (using quinine sulfate as reference) which is greater than that observed for pristine PQDs ( 83%). To account for this higher quantum efficiency of NPQDs, we carried out TCSPS measurement at 360 nm excitation wavelength, monitoring the emission at 420 nm. The comparative fluorescence decay profile is shown in figure 6. As evident from the decay profile, NPQDs shows a single exponential decay unlike that for pristine PQDs which exhibits a bi-exponential decay. Accordingly, the average lifetime (τ_av), radiative and non-radiative decay constants (k_r and k_nr) are calculated. The average lifetime of NPQDs is 874 ps which is lesser than that of pristine PQDs (956 ps). The k_r and k_nr are 10 × 10⁸ and 1.3 × 10⁸ s⁻¹, respectively, showing a significant enhancement in the radiative recombination rate and decrease in the non-radiative recombination rate after nitrogen doping. The higher rate of radiative recombination suggests less trap states. The recombination in the trap states will be non-radiative by nature (probability of electron-hole recombination without emitting photons is high in trap states) and hence a decreased non-radiative decay constant underscores decreased defects upon nitrogen doping in PQDs.

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The luminophores present in PQDs and NPQDs are distinguished by Fourier Transform Infrared Spectroscopy (FT-IR). Accordingly, figure 7 shows the comparative FT-IR spectra of PQDs and NPQDs. Both PQDs and NPQDs contain P = O and P-OH functional groups. Notably, P-NH₂ stretching and P-NH bending vibrational modes around 3400 and 1600 cm⁻¹ are noticed for NPQDs. A peak corresponding is P-N stretching vibration also appears at lower wavenumber region of the spectrum, which could be specially relevant to compare PQDs of various surface functionalization.

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The effect of nitrogen doping on PQDs is probed by the changes in the cyclic voltammograms (CV) of PQDs and NPQDs. Accordingly, figure 8 represents the superimposed solution CVs of PQDs and NPQDs obtained at 100 mVs⁻¹ on a glassy carbon electrode. A comparison of CVs of PQDs in a wide potential window of −2.5 V to +2.5 V in de-aerated propylene carbonate containing 0.1 wt % of LiClO₄ as the supporting electrolyte reveals several subtle differences. Apparently, two distinct redox couple could be identified for NPQDs although the extent of reversibility is very weak for PQDs with strong reduction peaks at −0.80 V and −2.0 V respectively. The electrode reactions of elemental phosphorus could be either reversible or irreversible in polar aprotic solvents [16]. The chemistry of BP can be looked at as continuous layers of P₄ molecules in which every P atom forms three bonds with its neighbouring P atoms [17]. Upon electroreduction, P-P bond cleaves and a reversible radical phosphido anion [P₄]^·− is likely to be generated via a one electron transfer (peak at −2.0 V). A broad anodic peak around −0.2 V is also noticed for PQDs. Interestingly this peak became prominent as a function of cycle number which indicates some surface reactions such as polymerisation possible at the electrode. The solution CV of NPQDs in a potential range of −3.0 to +3.0 V in acetonitrile containing 0.1 wt% tetraethylammonium tetrafluoroborate also follow the peak features of PQDs but with more kinetic reversibility. Interestingly an improved current density is also noticed which may be attributed to the increased conductivity up on doping. More studies using are in progress to obtain a clear understanding of the intermediates for electrochemical reduction and mechanism using in situ Electron Paramagnetic Resonance (EPR) Spectroscopy measurements. A comparison of the main properties of both PQDs and NPQDS is given in table 1.

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