Laser exfoliation of 2D black phosphorus nanosheets and their application as a field emitter

Abstract

Highly crystalline two dimensional (2D) few layered black phosphorus (BP) nanosheets have been synthesized via a one step facile laser irradiation technique under optimized experimental conditions. The field emission investigations on the few layered black phosphorus nanosheets were carried out at the base pressure 1 × 10⁻⁸ mbar. The morphological, elemental, optical, and structural analysis of the as-synthesized black phosphorus sample was carried out using SEM, AFM, EDAX, TEM, and Raman spectroscopy. The turn-on values of the BP nanosheets emitter were found to be significantly lower than that of earlier reports of BP nanosheets, graphene, and carbon nanotubes based field emitters due to the high field enhancement factor (β) ∼2986 associated with atomically thin/sharp edges of the BP nanosheets emitter. The emission current versus time plot depicts the good emission current stability with a pre-set value of 1 μA for ∼5 h duration. Our facile synthesis approach and the robust field emitter nature of the BP nanosheets makes them a potential candidate for a practical electron source in vacuum micro/nanoelectronic devices.

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1. Introduction

Two-dimensional graphene and its inorganic analogues, in particular the transition metal dichalcogenides (TMDCs) have continued to show great potential for applications in nanoscaled devices.^1–7 Presently, there is rising interest in exploring the anisotropic 2D nanostructure layered materials for various applications and consequently some TMDs like WS₂,⁸ MoSe₂,⁹ MoS₂,¹⁰ SnS,¹¹ and VS₂,¹² nanostructures have been investigated from a field emission (FE) point of view. The single layer black phosphorus (BP) has a honeycomb network similar to graphene, but is strongly puckered (armchair-shaped along x and zigzag-shaped along y axis).^13,14 Black phosphorous (BP) is the most thermodynamically stable allotrope of phosphorus and possesses high-charge carrier mobility. Optically BP has a direct band-gap dependent on the number of layers from 0.3 eV (for bulk) to 2.0 eV (single layer). As a result, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared opto-electronics.^15,16 Hence, owing to a set of exotic physico-chemical properties including, very high carrier mobility,¹⁷ quantum confinement in nanoscale dimensions,¹⁸ single to few layer tunable transport,¹⁹ etc., have attracted BP materials as an great deal of attention of researchers worldwide. Superior FE behaviour of various nanomaterials has opened up a door to new application, one of them being efficient cold cathodes. The atomically thin 2D layered BP nanosheets possess atomic sharp edges offering high aspect ratio, along with unique set of electronic and mechanical properties. These features are important for deciding the emission behavior and, thus there is scope to investigate its FE characteristics of BP nanostructures/nanosheets. Furthermore, the FE performance of different nanomaterials have been improved by attempting plasma etching,²⁰ photo-switching,²¹ or exposing the emitter to specific ambient conditions during the FE measurements.²² These BP nanosheets need to be deposited in thin film (planar emitter) so as to investigate the FE behaviour and further to develop cathode materials for practical applications. The FE being extremely surface sensitive phenomenon, preparation of such planar emitters is very important aspect of cathode development. Here, in this article we have synthesized few layer BP nanosheets by optimized laser irradiation method and investigated their FE characteristics in ultra high vacuum. The results obtained reveal that BP nanosheets based planar emitter has potential to be used in FE based devices.

2. Experimental methods

2.1 Synthesis of few-layer BP nanosheets

Bulk crystals of BP were purchased from Smart Elements GmbH Ferrogasse 4/I A-U80 Wien GERMANY (purity 99.998%). Isopropyl alcohol (IPA) was purchased from Chemlabs (Leonid Chemical Pvt. Ltd, Bangalore, India). Typically, 13 mg of the bulk crystal of BP was crushed into small pieces by mortar and pestle with 5 ml of isopropyl alcohol (IPA). Later this sample was transferred to a quartz beaker and then dispersed in IPA (concentration ∼ 0.5 mg mL⁻¹). Immediately after transferring the dispersion the quartz beaker was closed with a quartz optical window and sealed with parafilm and Teflon tapes. The quartz beaker was equipped with ports to maintain Ar ambient during the laser irradiation. The Krypton fluoride (KrF) laser with a wavelength of 248 nm, spot size ∼ 80 mm² was used for the irradiation of the dispersion at a fluence 90 mJ cm⁻² and a frequency of 10 Hz. The laser irradiation was carried out for 1 hour at normal incidence (relative to top of the beaker). During the laser irradiation, the dispersion was continuously stirred in order to make sure that the BP crystallites do not settle down. After the laser irradiation for 1 h we could observe the thin BP sample floating at the top of dispersion and remaining portion of the BP settled down at the bottom of the beaker. An extreme care has been taken while collecting the supernatant of BP from IPA solution. Afterwards the sample were centrifuged at 10 000 rpm and dried in vacuum oven at 80 °C for 4 hours. The BP sample was removed from the vacuum furnace, which showed presence of black product in the form of powder. Fig. 1(a) shows the typical schematic of the side view of BP nanosheet. The schematic of the experimental set up used for the laser irradiation is shown in the Fig. 1(b). The as synthesized BP powder was characterized using scanning electron microscopy (SEM, JEOL 6360A) and the elemental composition was obtained from the EDAX (energy-dispersive analysis of X-rays) spectrum. The Atomic Force Microscopy (AFM) images and AFM height profile were carried out using an ICON system (Bruker, Santa Barbara Ca.) in tapping mode. Further, the surface morphology and crystallographic fractures were confirmed with the help of transmission electron microscopy (TEM, FEI TECNAI G2 F-20 FEG). The structural analysis was carried out using Raman spectroscopy (LabRAM HR Horiba Jobin Yvon, 632.8 nm laser source) in the standard back scattering geometry at a power of 200 μW. For TEM analysis the sample were prepared by drop casting the BP nanosheets dispersed in ethanol onto a TEM grid (mesh 200 μm). For the SEM characterization the samples were prepared by drop casting the BP nanosheets dispersion in ethanol onto a pre-cleaned silicon substrate and the same was used for the Raman spectroscopy characterization.

Fig. 1 (a) Schematic showing the side views of BP nanosheets. (b) Schematic of the experimental set up used for the synthesis of BP nanosheets by laser irradiation of bulk crystal.

2.2 Field emission investigation of few-layer BP nanosheets

The FE current density (J) versus applied electric field (E) and emission current (I) versus time (t) characteristics were measured in a planar ‘diode’ configuration at base pressure of ∼1.0 × 10⁻⁸ mbar. A typical ‘diode’ configuration consists of a phosphor coated semitransparent screen (a circular disc having diameter ∼ 40 mm) as an anode and the sample under study as cathode. In order to investigate the FE properties, BP nanosheets were sprinkled onto a piece of carbon tape (0.5 cm × 0.5 cm). This BP sprinkled carbon tape was pasted on a stainless steel holder (diameter ∼ 4.5 mm), which acted as a cathode. The FE measurements were carried out at fixed cathode–anode separation of ∼1 mm. The ultra high vacuum (UHV) system comprises of turbo molecular pump backed by a rotary pump, a sputter ion pump and titanium sublimation pump with liquid nitrogen jacket. The field emission current was measured on Keithley electrometer (6514) by sweeping dc voltage applied to cathode with a step of 40 V (0–40 kV, Spellman, U.S.). The emission current stability was studied at pre-set current value of 2 μA. Special care has been taken to avoid any leakage current using shielded cables and ensuring proper grounding. Before recording the FE current–voltage (I–V) measurements, pre-conditioning of the cathode was carried out by keeping it at ∼800 volts for 15 min so as to remove loosely bound particles and/or contaminants by residual gas ion bombardment.

3. Results and discussion

The SEM images of the bulk BP crystal and laser exfoliated samples are presented in Fig. 2(a)–(c). Fig. 2(a) represents the SEM image of the pristine BP bulk crystal, where a big chunk characterized with ‘layered’ surface is observed. Interestingly, upon laser irradiation, formation of micron sized BP sheets is observed as depicted in Fig. 2(b) and (c). The BP sheets stack with each other and these stacks exhibit random orientation. The laser irradiation of bulk BP sample was carried repeatedly and results are found to be similar in nature. The SEM image recorded at higher magnification (Fig. 2(c)) clearly reveals sharp edges of the BP sheets. Compositional analysis of the BP nanosheets was performed using EDAX spectrum, which indicated presence of P in the as-synthesized product (Fig. 2(d)).

Fig. 2 SEM micrograph of the bulk BP crystal (a) before laser irradiation and (b and c) after laser irradiation. (d) EDAX spectrum of the BP nanosheets.

Further, the AFM analysis of as synthesized BP nanosheet sample was carried out to find out the thickness and surface morphology as shown in Fig. 3(a) and (b). The AFM analysis confirms the typical height of BP nanosheet ∼7 nm. In order to gain better morphological and structural insight, TEM analysis was carried out. Fig. 4(a) and (b) depicts the typical bright field TEM images of the as-synthesized (laser exfoliated) BP sample, revealing formation of randomly oriented nanosheets. Although the overall size of the BP sheet is observed to be of micron meter square, it is being imaged in TEM indicates that its thickness is very small as compared to the length and breadth. The selected area electron diffraction (SAED) pattern (Fig. 4(c)) depicts six-fold symmetry, indicative of crystalline nature of the BP nanosheets. Fig. 3(d) shows the HRTEM image of the BP nanosheet with observed ‘d’ spacing of ∼0.25 nm, which corresponds to (221) plane.

Fig. 3 (a) Typical AFM image of few-layer black phosphorus nanosheets and (b) corresponding AFM height profile.

Fig. 4 TEM images of the BP nanosheets (a) low magnification bright field image (b) high magnification bright field image (c) selected area electron diffraction (SEAD) pattern and (d) HRTEM image.

Fig. 5 shows the Raman spectra of as synthesized nanosheets and pristine samples. The Raman spectrum of pristine BP crystal exhibits well defined peaks at 362.23, 437.07, and 464.75 cm⁻¹ corresponding to the phonon modes A¹_g, B_2g and A²_g, respectively, and is in good agreement with literature.^21–23

Fig. 5 Raman spectra of pristine and laser exfoliated BP nanosheets samples.

Interestingly, the Raman spectrum of the laser exfoliated sample exhibits the identical structural features seen at 363.69, 440.12 and 467.60 cm⁻¹ corresponding to the A¹_g, B_2g and A²_g phonon modes, respectively. A careful analysis of the spectra reveals blue shift in peaks positions along with small variation in their full width at half maximum (FWHM) values. The observed blue shift upon laser exfoliation is attributed to the reduced thickness of the BP nanosheets.^23–25

The laser induced mechanism of nanomaterials (sheets) formation mainly involves two processes; the first process is laser illumination produces the thermal shock. Due to the thermal shocks, the ‘stresses’ produced if not relieved via melting, then would lead to cracking and flake detachments from the bulk. This detachment of ‘flakes’ from bulk is termed as laser exfoliation. It is highly expected that laser exfoliation caused the septules of bulk BP into nanosheets. In the second process, there is finite probability (although very small) that the expelling flakes may be hit by the shock waves produced during subsequent pulses, causing further diminution to particles (as observed bright field TEM image, small size BP particles on nanosheets).²⁶

Moreover, for the dissociation of IPA molecule 10 eV energy is sufficient to ionization which is equal to the energy of two 248 nm photons (∼10 eV). In case of inorganic layered materials, which are bound together by week van der Waals force of interaction, which is easy to break the bonding by laser irradiation or mechanical exfoliation. In present case laser energy of ∼10 eV is sufficient to ionization which is equal to the energy of two 248 nm photons (∼10 eV). Therefore, the two-photon excitation of IPA under UV light illumination leads to the generation of reactive water radicals, such as C₃H₇O_aq⁻ and H_aq⁻ radicals and hydrated electrons (e_aq⁻). Those photodissociation products can act as intercalants for bulk BP materials and lead to exfoliation it in IPA. J. J. Hu et al. have reported synthesis of laser induced layered WS₂ nanoparticles in water.²⁴

The dependence of the FE current density versus applied electric field (J–E) of the BP nanosheets (laser exfoliated) emitter is depicted in Fig. 6(a). The values of the turn-on and threshold field, defined as field required to draw an emission current density of 1 and 10 μA cm⁻² were found to be 2.1 and 2.3 V μm⁻¹, respectively. Interestingly, higher emission current density of ∼978 μA cm⁻² has been drawn from the BP nanosheets emitter at a relatively lower applied electric field of ∼3.9 V μm⁻¹, indicating its superior performance. As the applied electric field is gradually increase, the emission current density is observed to increase very rapidly, indicating that the emission is as per the quantum mechanical tunneling of electron through the surface potential barrier.²⁵ The present results demonstrate that, the turn-on and threshold values of BP nanosheets emitter are comparable to those observed for various nanostructure field emitters. Table 1 shows the comparison of the FE results of present study and other nanostructure field emitters.

Fig. 6 Field emission characteristics of the BP nanosheets emitter, (a) field emission current density versus applied electric field (J–E) curve, (b) Fowler–Nordheim (F–N) plot, (c) field emission current versus time (I–t) plot. (d) Typical field emission pattern recorded at current density of ∼250 μA cm⁻².

Table 1 Turn-on field values of BP nanosheets and various other field emitters reported in the literature

Field emitter	Turn-on field (V μm^−1)	Field enhancement factor (β)	Reference
MoS2 nanoflowers	4.5–5.5	—	10
Graphene	5.8	—	29
Ga2O3 nanowires	6	880	32
GdB6@Cu2O heteroarchitecture	2.3	2860	32
BP nanosheets	3.8	2891	30
	5.1	1164	31
BP nanosheets	2.1	2986	Present work

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As revealed from the SEM images, most of the black phosphorous nanosheets are found to be nearly vertically aligned, which act as potential emitting sites. Furthermore, the areal density of the BP nanosheets is also seen to be reasonably good, thus permitting the less screening of the electric field between adjacent emitting sites (nanosheets). Thus, the observed superior values of turn-on and threshold field are attributed to the observed high aspect ratio of nanosheets emitter, smaller areal density of the emitters, less screening effect of the BP emitter. Therefore, BP nanosheets based emitter is attractive candidates for nanoelectronic and vacuum microelectronic devices based on field emission.

The modified Fowler–Nordheim (F–N) equation for atomically emitting sites deposited on flat substrate is given as,²⁷

where J is the emission current density, E is the applied average electric field, a and b are constants, typically 1.54 × 10⁻¹⁰ (A V⁻² eV) and 6.83 × 10³ (V eV^−3/2 μm⁻¹), respectively, ϕ (5.2 eV for BP)²⁸ is the work function of the emitter material, λ_m a macroscopic pre-exponential correction factor, ν_F is value of the principal Schottky–Nordheim barrier function (a correction factor), and β is the field enhancement factor. For the planar emitter, the applied electric field (E) is defined as E = V/d, V is the applied voltage, and d is the separation between cathode and anode.

The field enhancement factor (β) is the ratio of local electric field to applied electric field. The J–E characteristic is further analyzed by plotting a graph of ln(J/E²) versus (1/E), known as a Fowler–Nordheim (F–N) plot. The F–N plot depicted in Fig. 6(b) exhibits overall non-linear behavior indicative of the semiconducting nature of the emitter. Such non-linear F–N plots have been reported for various semiconducting nanostructure emitters.^1,2,8,9,25

The deviation from the linearity to non-linearity was due to BP sample being multi emitter type, semiconducting in nature and other effect such as band bending, screening effect, field penetration etc. The slope (m ∼ −27.02) of the F–N plot is used to estimate the value of field enhancement factor (β). The field enhancement factor calculated from the F–N equation was found to be ∼2986. The high field enhancement factor was attributed to the edges, corners, well-aligned and freely standing BP nanosheets.

Fig. 6(c) shows the emission current versus time (I–t) plot corresponding to preset value of ∼2 μA, recorded for sampling interval of 10 s over an entire period of 5 hours at a base pressure of 1 × 10⁻⁸ mbar. For the practical application of the field emitters mostly requires the stable operation of the cathodes. The present BP based field emitters exhibits good emission stability at 2 μA and the main cause of ‘excursion’ type current fluctuations in the field emission stability of the BP nanosheets are attributed due to field induced adsorption, or desorption of the residual gas molecules on the emitter surface. More interestingly, the average emission current is seen to remain constant over the entire duration, which is described to the good robustness of the emitter. Fig. 6(d) shows typical field emission pattern recorded at an emission current density of ∼250 μA cm⁻², indeed emission is mostly from the protruding edges of the black phosphorous nanosheets. This is a remarkable and promising feature of BP nanosheets emitters particularly useful in practical device application as electron source.

4. Conclusions

In conclusion, highly crystalline black phosphorus nanosheets have been synthesized by optimized laser irradiation method from the bulk crystal. The field emission characteristics of the BP nanosheets emitter in terms of turn-on and threshold field values are found to be superior to those reported for other chalcogenides nanostructures. Furthermore, high emission current density of ∼978 μA cm⁻² was drawn from the BP nanosheets emitter at lower applied electric field of ∼3.9 V μm⁻¹. The emitter exhibits stable emission current at pre-set value of ∼2 μA over a longer duration. The overall superior field emission characteristics propose that the BP nanosheet based field emitter as a promising electron source for practical applications in various future vacuum micro/nanoelectronic devices.

Acknowledgements

Authors would like to thank Prof. C. N. R. Rao (FRS), JNCASR and ICMS Bangalore (India) for encouragement and support. The research work was supported by Department of Science and Technology (Government of India) under Ramanujan Fellowship to Dr D. J. Late (Grant No. SR/S2/RJN-130/2012), NCL-MLP project grant 028626, DST-SERB Fast-track Young scientist project Grant No. SB/FT/CS-116/2013, Broad of Research in Nuclear Sciences (BRNS) (Government of India), Grant No. 34/14/20/2015 and partial support by INUP IITB project sponsored by DeitY, MCIT, Government of India and CINT Proposal #U2015A0083 (USA). Authors thank Director, CSIR-NCL, Dr Sucharita Sinha (L & PTD, BARC, Mumbai) and Prof. S. B. Ogale (IISER, Pune) for the help and support.

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