Highly efficient field emission properties of vertically aligned 2D CuSe nanosheets: An experimental and theoretical investigation

We report the synthesis of klockmannite (CuSe) via a three-probe electrochemical set-up (chronoampereometry). The structural properties are examined by X-ray diffraction and Raman spectroscopy. Field emission scanning electron microscopy (FESEM) analysis revealed the formation of vertically aligned CuSe nanosheets with an average thickness of 34 nm and an average lateral size of 700 nm. The CuSe nanosheets exhibit impressive field electron emission characteristics with a turn-on field of 1.4 V/µm for 10 µA/cm 2 emission current density. Also, a high current density of 5.8 mA/cm 2 is observed at a relatively low applied field of 3.1 V/µm. Complementary first-principles DFT calculations show that CuSe displays metallic conductivity, and the (001) surface has a low work function of 5.12 eV, which is believed to be responsible for the impressive field emission characteristics.


Introduction
In comparison with bulk materials, nanomaterials are popular for phase engineering as different phases of nanomaterials can be tuned with synthesis parameters for optimal performance [1,2].The precise control over the synthesis of unconventional phases of nanomaterials has opened the door for various correlation studies between a structure, properties and appropriate applications.Rather than a routine study of core structures of nanomaterials and their applications in recent decades, many structural developments/ modifications in nanomaterials have been carried out for particular applications such as magnetic [3], electronic [4], mechanical [5], catalytic [6], and optical devices [7].
Various synthesis techniques have been explored for the synthesis of CuSe nanomaterials including the chemical aqueous route [15], hydrothermal method [16], solvothermal method [17] and hot injection method [18].However, reports on the synthesis of CuSe in the klockmannite phase are rather limited [19].CuSe is popularly known for various potential applications such as photo-detectors [20], Shottkydiodes [21], solar cells [22] and field emission (Cu 1−x Se) [23].
Field electron emission is the fundamental process of electron ejection from a material surface which occurs under the application of a high electric field [24].Field electron emission capabilities of various one-dimensional (1D) and two-dimensional (2D) nanomaterials have been explored in previous reports [25][26][27][28][29][30][31].In particular, the 2D nanomaterials have been demonstrated to exhibit superior field emission properties in terms of turn-on field, high current density and emission stability [27].Although CuSe has been studied for electronic and optoelectronic applications, the suitability of 2D CuSe nanosheets for field emission applications has not been investigated thoroughly.
Herein, we report a simple one-pot synthesis of vertically aligned, single crystalline, 2D CuSe nanosheets and subsequent field emission characteristics.In addition, theoretical investigation of the electronic structure and work function of CuSe have been carried out using first-principles density functional theory calculations.

Synthesis
The synthesis of vertically aligned 2D CuSe nanosheets is done using a chronoampereometry technique.Fig. 1 shows a schematic of the experimental synthesis set-up.Stainless steel (SS) foil is used as the substrate throughout.CuCl 2 :2H 2 O (2 mM), SeO 2 (4.5 mM) and KCl (0.1 mM) are combined in distilled water to make the electrolyte solution.0.2 M HCl was added to adjust the electrolyte pH to 1.5.For the three-probe electrode system, SS foil, platinum (Pt) and calomel electrode (SCE) were used as the working, counter and reference electrodes, respectively.The vertically aligned 2D CuSe nanosheets were prepared via a potentiostatic approach.At the time of deposition, the applied potential was −0.15 V (vs SCE) and the temperature was kept constant at 60 °C for 30 min.

Materials characterization
X-ray diffraction studies were carried out by Bruker (D8 Advance) for a scan rate of 1°/min.A Field Emission Scanning Electron Microscope (FESEM) model-Quantum 450 FEG was used to examine the morphology and surface topography of the vertically aligned 2D CuSe nanosheets.The morphology and crystallinity of the as-prepared samples were studied by Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HR-TEM) and Selected Area Electron Diffraction (SAED) (model number-JEOL 2200 FS, source-FEG 200 kV).Elemental mapping analysis was done using Energy-dispersive X-ray spectroscopy (EDS).The samples were dry dispersed over 300 mesh copper grids coated with holey carbon film.Field emission measurements such as the log of current density-applied field (log(J)-E) and current-time (I-t) were carried out at stable pressure ~1 × 10 -8 mbar.The planer diode i.e., 'close proximity' configuration was employed during field emission studies [32].In the present case, the area of CuSe specimen was 0.25 cm 2 and the separation between the anode and cathode was 1 mm.Here, the vertically aligned 2D CuSe nanosheets film was directly pasted onto sample holder using a highly conducting carbon tape.Ultra-high vacuum (UHV) was achieved with the help of turbo molecular pump and rotary pump.Above mentioned pressure (~1 × 10 -8 mbar) reached after baking the system at 150 °C for 12 h.Here, the applied electric field is, = E V d , where V applied voltage and d is distance separated by anode and cathode.

Computational details
The density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) [33][34][35], a periodic plane-wave DFT code.The interactions between the core and valence electrons was described using the Project Augmented Wave (PAW) method [36].A plane-wave basis set with a kinetic energy cut-off of 500 eV was used to converge the total energy of CuSe to within 10 −6 eV.The Brillouin zone of bulk CuSe was sampled using a 7 × 7 × 3 Monkhorst-Pack k-point mesh [33].All calculations were deemed to be converged when the forces on all atoms reached 0.001 eV/Å.The electronic exchange-correlation potential was calculated using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional [34].The electronic structure (total and projected density of states) was determined using the screened hybrid functional HSE06 (α = 0.25 and ω = 0.11 bohr −1 ) [35].The most stable CuSe(001) surface has been used to characterize the work function of CuSe.Previous theoretical calculations have predicted the lowest surface energy for the (001) surface of CuS, which is isostructural to CuSe [37,38].In order to align the energies to the vacuum level, a slab-gap model (slab thickness of 17.259 Å and vacuum size of 20 Å) was constructed and the corresponding electrostatic potential was averaged along the c-direction, using the Macro Density package [39][40][41].The work function (Φ) was calculated as Φ = V vacuum ─E F, where V vacuum and E F are the vacuum and Fermi level, respectively.

Structural properties
The X-ray diffraction spectrum of the vertically aligned 2D CuSe nanosheets is shown in Fig. 2a.The peaks observed at 25.11°, 26.61°, 28.01°, 31.32º and 49.86° can be indexed to CuSe klockmannite phase, with JCPDS card number #34-0171.The other peaks correspond to the SS substrate and are indexed by *.These peaks originate from C and Fe elements [42,43].

Surface morphological properties
FESEM analysis confirms the surface topography of the CuSe nanosheets as shown in Fig. 2(b-d).The high magnification image showed in Fig. 2b reveals a large coverage of nanosheets and a close up image in Fig. 2c shows vertically aligned nanosheets of CuSe.The average thickness of the nanosheets is about 34 nm with the average lateral size estimated at 700 nm (Fig. 2c).TEM characterization (Fig. 3a) further confirms the 2D nanosheet-like morphology of CuSe.The HRTEM image in Fig. 3b reveals the single crystalline nature of CuSe with fringe spacing of 0.34 nm [44].The inset of Fig. 3b is the SAED pattern, which confirms that the sample is single crystalline.The SAED pattern recorded perpendicular to the (001) surface that could be attributed to the [001] zone-axis diffraction of the hexagonal phase of CuSe.The fuzzy spots may be due to the line defect and overlapping of CuSe nanosheets.The bright TEM image (Fig. 3c) is employed for elemental mapping analysis, which shows an equal distribution of C, Cu and Se elements throughout the 2D CuSe nanosheet as shown in Fig. 3(d-f).

Field emission studies
The field emission properties of the vertically aligned 2D single crystalline CuSe nanosheets have been expounded by the way of log of current density versus applied field (log(J)-E), Fowler-Nordheim (F-N) and emission current stability versus time (I-t) plots.The turn-on field, defined in the present study as the minimum field required to withdraw an emission current of 10 µA/cm 2 is found to be 1.4 V/µm (inset of Fig. 4a).The maximum current density withdrawn from the emitter material is observed to be 5.8 mA/cm 2 at an applied electric field of 3.1 V/µm.Table 1 shows the comparison between the turn-on field values of selenium based nanomaterials and the vertically aligned 2D CuSe nanosheets [45][46][47][48][49][50][51][52]29].The field enhancement factor (β) [53] has  , where, ϕ is work function.The value of β is calculated at 3545.The overall nature of observed F-N plot is seen to be non-linear.For the calculation of β, higher field region of F-N plot has been chosen and it is shown in Fig. 4b.Possible reasons behind the observed impressive field emission characteristics are suggested to be due to the sharp edges, vertically aligned morphology, high β value and the metallic nature of CuSe [28].According to SAED indexing and owing to the stability of the (001) surface, it is expected to be the most expressed and abundant facet in CuSe nanocrystals.Thus, the field emission is suggested to come from the most stable and most expressed (001) surface.The vertically aligned 2D CuSe nanosheets display a quite stable emission current (10 µA at 2 kV) recorded over a 2 h period of testing (Fig. 4c).As the field emission is a surface sensitive phenomenon, the small fluctuations observed in I-t plot during initial period of measurement may be credited to the adsorption/desorption of residual gases [29].

Density functional theory (DFT)
In view of the fact that field emission characteristics are dependent on the work function (Φ) of the emissive material, we have carried out electronic structure calculations based on density functional theory (DFT) to gain atomic-level insight into the electronic structure and work function of CuSe.The bulk klockmannite CuSe was modeled in the hexagonal crystal structure (space group -P6 3 /mmc) [54,55] as shown in Fig. 5a.The (001) surface is also predicted to be the most stable for CuSe, which is isostructural to CuS.Therefore, the CuSe (001) surface is chosen for the DFT calculations.The fully relaxed unit cell parameters were predicted at a = 3.980 Å and c = 17.259Å, which compares closely with experimental data and previous theoretical structure of hexagonal CuSe [54,55].The metallic characteristic of CuSe [56] is also well reproduced, as shown by the electronic projected density of states (PDOS) in Fig. 5b, with the Se atoms contributing a higher proportion of density of states at the Fermi level than Cu atoms.This is consistent with the metallic behavior exhibited by the isostructural covellite CuS [57].The optimized structure of the CuSe(001) surface employed to characterize the work function is shown Fig. 5c, with the corresponding electrostatic potential shown in Fig. 5d.The work function of the CuSe(001) surface is 5.12 eV versus vacuum.The gas molecules that may adsorb on the sample surface during field emission experiments, could further decreases the work function owing to the adsorption induced electron density redistribution in the gas-covered sample surfaces [57].As the work function dictates the electron emission capability of a the emitter material, the low work function calculated for the CuSe(001) is suggested as the primary origin for the impressive field electron emission characteristics observed in Fig. 4.

Conclusion
In summary, we have employed a non-hazardous, economical and simple technique for synthesis of vertically aligned single crystalline CuSe nanosheets.The 2D CuSe nanosheets are demonstrated to show impressive turn-on field of 1.4 V/µm for 10 µA/cm 2 emission current density and a high emission current density of 5.8 mA/cm 2 at an applied electric field of 3.1 V/µm.The CuSe nanomaterials exhibit metallic conductivity and a low work function of 5.12 eV for the CuSe(001) surface as predicted from first-principles DFT calculations.The observed excellent field emission characteristics of single crystalline CuSe nanosheets may be due to sharp edges, vertically aligned nature, high β value, metallic conductivity, and low work function.Our results show that the controlled synthesis of 2D CuSe nanosheets may open new avenues for further development towards efficient field emission applications.

Fig. 3 .
Fig. 3. (a) TEM image, (b) HRTEM image with fringe spacing and inset SAED pattern, (c) bright TEM image for elemental mapping, (d-f) elemental mapping images for C, Cu and Se elements with colors red, green and blue respectively.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Chandradip D. Jadhav and Sachin R. Rondiya: performed synthesis, field emission studies, characterizations, and wrote manuscript.Padmakar G. Chavan: contributed to the study design and scientific discussions on methods and conclusions.Nelson Y.

Fig. 5 .
Fig. 5. (a) Crystal structure of CuSe, (b) the partial density of states (PDOS), (c) the optimized CuSe(001) surface in the side view and (d) the corresponding electrostatic potentials for the CuSe(001) surface.The black and red dashed lines represent the vacuum level (E vacuum ) and the Fermi level (E Fermi ), respectively.The Φ is the work function.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1
Comparison of turn-on field with reported selenium based nanomaterials (for anodecathode distance d =1 mm).
been calculated by using an F-N plot based on the formula: