Vertically aligned diamond-graphite hybrid nanorod arrays with superior field electron emission properties

A “patterned-seeding technique” in combination with a “nanodiamond masked reactive ion etching process” is demonstrated for fabricating vertically aligned diamond-graphite hybrid (DGH) nanorod arrays. The DGH nanorod arrays possess superior field electron emission (FEE) behavior with a low turn-on field, long lifetime stability, and large field enhancement factor. Such an enhanced FEE is attributed to the nanocomposite nature of the DGH nanorods, which contain sp2-graphitic phases in the boundaries of nano-sized diamond grains. The simplicity in the nanorod fabrication process renders the DGH nanorods of greater potential for the applications as cathodes in field emission displays and microplasma display devices.

based hard masks can create non-uniformities and metallic contamination in the patterned diamond nanostructures.
In this study, we reported the fabrication of diamond-graphitic hybrid (DGH) nanorod arrays via a combination of a novel "patterned-seeding" technique for selective area growth of nanocrystalline diamond (NCD) films and a "nanodiamond-masking" technique for reactive ion etching (RIE) of NCD films.The vertically aligned DGH nanorod arrays thus obtained show a much better FEE performance in comparison with the other kind of diamond nanostructures that is pragmatic to be implemented for large scale device applications.
Figure 1 illustrated the schematic diagram for the fabrication of vertically aligned DGH nanorod arrays.Dielectric coated n-type (100) silicon substrates, which were undergone chemical surface modification, 13 are seeded using a colloidal suspension containing ND particles (∼5 nm in diameter) in deionized water [Fig.1(a)]. 14Next, a negative photoresist (NR7-3000P; Futurrex, Inc.) is spin coated and patterned using contact lithography using a Carl Suss MJB3 UV mask aligner to obtain isolated circular micro-patterns [Fig.1(b)], where the unexposed regions of the photoresist were removed using an MAD-420 developer.The patterned substrates were then exposed to plasma oxidation using a home-built DC-pulsed sputtering/oxidation system for about 5 min at 50 W power and 4.5 mbar pressure to remove excess ND seeding particles from the unpatterned regions, leaving behind photoresist patterns containing ND particles [Fig.1(c)].
The substrates are then transferred into a microwave plasma enhanced chemical vapor deposition (MWPECVD, ASTeX 6500) reactor for the growth of patterned NCD films, which have the same pattern as the PR cylinders [Fig.1(d)].The NCD films were deposited using a CH 4 (6%)/H 2 (89%)/N 2 (5%) plasma with a microwave power of 3000 W, where 5% N 2 was added in CH 4 /H 2 plasma to achieve nano-sized diamond grains for NCD films. 15The flow rate and the pressure were maintained at 300 SCCM and 40 mbar, respectively, during the growth of NCD films.The substrates were heated up due to the bombardment of the plasma species and the growth temperature was estimated to be around 540 °C, which was measured in situ using an infrared optical fiber pyrometer (Williamson PRO 9240C, USA).The NCD deposition was carried out for 4 h to reach a thickness of 1.5 µm.Figures 2(a It should be noted that the "pattern-seeding" technique used in this study is a relatively efficient way for forming NCD pads.Patterns with feature size as small as 0.5 µm and with a sharp edge can be formed [cf.Figs.2(a) and 2(b)].However, to successfully pattern NCD pads using such a "pattern-seeding" technique, tuning of the zeta potentials of the substrate surface is required to ensure a uniform seeding density.Moreover, the choice of photoresist materials is very critical.The photoresist materials should be able to survive the plasma oxidation process for removing the residue seeding ND particles [cf.Fig. 1(c)], yet they should be readily dissociable under a MWPECVD process for selective area growth of NCD films.Small debris occasionally formed due to unselective growth [indicated by arrows in Fig. 2(a)] can be completely removed in the followed RIE process for fabricating diamond nanorod arrays [cf.Figs.2(c) and 2(d)].This technique is simpler and more reliable compared with the conventional lift-off process and is especially useful for the fabrication of miniaturized emitters such as multi-figure lateral FEE emitters, which are key components for vacuum triode transistors. 16igure 3(a) shows that the patterning process for fabricating NCD pads efficiently improved the FEE properties of NCD films.FEE characteristics of the samples were measured using a planeto-plane electrode geometrical setup [inset of Fig. 3(a)], with a Mo rod about 2 mm in diameter used as an anode and diamond samples as a cathode.The cathode-to-anode separation was controlled to be around 66 µm by a micrometer.For such a small cathode-to-anode distance, the rod-shaped anode is large enough to neglect the fringing field effect, i.e., the anode and cathode are essentially lying in parallel with each other.The current-voltage characteristics were acquired using a Keithley 2410 electrometer (Keithley Instruments, Inc., OH, USA).The FEE behavior of the materials was modeled using Fowler-Nordheim (F-N) theory, 1,17 βE , where A = 1.54 × 10 6 A eV/V 2 and B = 6.83 × 10 9 eV 3/2 V/m, β is the field enhancement factor, E is the applied field, J is the FEE current density, and ϕ is the work function of the emitting materials.The turn-on field (E 0 ) for triggering the FEE process was designated here as the lowest value of the F-N plots, ln J/E 2 -1/E curves, corresponding to the intersection of the low and high electric field segments.The slope of F-N plots, ln J/E 2 -1/E, can be represented by m = 6830 ϕ 3/2 / β, from which the β-values of the emitting materials can be estimated.While the planar NCD films grown APL Mater.5, 066102 (2017) directly on Si substrates show large turn-on field (E 0 ) of 24.40 V/µm [curve III, Fig. 3(a)], the FEE process for NCD pads can be turned on at 10.42 V/µm [curve II, Fig. 3(a)].Moreover, the β-values of these FEE emitters were estimated as β NCD pads = 1950 and β planar NCD = 580 [curves II and III of Fig. 3(b)], by taking the ϕ value as 3.5 eV. 18,19The enhanced FEE properties of patterned NCD pads compared with planar NCD films are apparently due to the geometric factor, which induced local field enhancement, that is, the electrons are primarily emitted from the edge of the "micron" sized circular pads.The circular NCD pads contain abundant edges on the top of the surface that produce large field concentration and enhance the FEE properties of NCD pads. 20,21o further improve the β value for the NCD emitters, the DGH nanorods were fabricated from NCD pads.For this purpose, the NCD pads were immersed in a pseudo-stable suspension of ND particles (∼5 nm in diameter) in de-ionized water and sonicated for 10 min to adhere ND particles on the patterned NCD pads surface to serve as a mask for the RIE etching process [Fig.1(e)].The number density of ND particles on the NCD pads depends on the suspension characteristics (such as concentration and viscosity) and time of sonication.After masking, the patterned NCD pads were then etched using the RIE process in O 2 gas at a RF power of 200 W for 30 min using the homebuilt DC-pulsed sputtering/oxidation system [Fig.1(f)]. 22Figures 2(c) and 2(d) illustrate the SEM images of the patterned DGH nanorod arrays thus obtained.It can be seen clearly from Fig. 2(d) that the patterned DGH nanorods are distinct, uniform, and densely populated.An enlarged tilted SEM image reveals the presence of vertically aligned DGH nanorods [inset of Fig. 2(d)].Notably, the fabrication of nanostructures from diamond materials, which are extremely hard and chemically inert materials, is a very difficult task.Lots of efforts were made in exploring the possible techniques for fabricating the DGH nanorods.Eventually a simple recipe for fabricating the DGH nanorods array from NCD pads was achieved, i.e., the utilization of ND particles as a mask in the O 2 plasma RIE process.
The proposed formation mechanism of DGH nanorods is as follows: generally, the NCD film contains nanodiamond grains (∼80 nm) separated by the grain boundaries. 15The grain boundaries contain a large proportion of sp 2 -carbon and amorphous carbon (a-C) phases, which are susceptible to O 2 plasma etching than the diamond materials.At the initial stage of the etching process, ND particles sitting on top of the NCD pads mask the NCD films from the ion bombardment.The chemical etching of oxygen ions starts from the grain boundaries of non-masked region of the NCD pads.Because of the ease of etching on the sp 2 -bonded and a-C from the boundaries of NCD grains, an etching path for shaping the nanorods is created, resulting in vertically aligned nanorods. 23,24The etching process was stopped (by trial and error) once the masking ND particles were also etched away by the O 2 plasma so as to prevent the plasma damage on the formed nanorods.Such a technique is similar to Yang's process, 25 which utilized ND particles as a mask in a RIE process for fabricating the diamond nanorods from ultrananocrystalline diamond films.
The FEE behaviors of the patterned DGH nanorod arrays are shown as curve I in Fig. 3(a), revealing the fascinating FEE properties for these materials.Extremely low E 0 value of 5.26 V/µm is attained for patterned DGH nanorod arrays, which is markedly smaller than the E 0 values of NCD pads [(E 0 ) = 10.42V/µm; curve II, Fig. 3(a)] or planar NCD films [(E 0 ) = 24.40V/µm; curve III, Fig. 3(a)].It needs an applied field of (E) DGH nanorods = 10.55 V/µm to reach a large FEE J value of 1.0 mA/cm 2 for patterned DGH nanorods [curve I, Fig. 3(a)].This applied field is smaller than those for NCD pads [(E) NCD pads = 20.02V/µm; curve II, Fig. 3(a)] and planar NCD films [(E) planar NCD = 37.17 V/µm; curve III, Fig. 3(a)].Interestingly for the patterned DGH nanorods, two slopes are observed in the F-N curves as shown in curve I of Fig. 3(b), while no such change is observed in the other two samples.By assuming the ϕ value of 3.5 eV, 18,19 the β values corresponding to the two slopes in the F-N plot are calculated to be 3270 (and 2800) at high (and low) electric field regimes, respectively.
The better FEE behavior for DGH nanorod arrays, as compared with the NCD pads and the planar NCD films, can apparently be ascribed to higher β-values for the DGH nanorod arrays, as all of the emitters are made of the same diamond materials.However, the two slopes characteristics in the F-N plot for DGH nanorod arrays, in contrast to the single F-N slope for the other two FEE materials cannot be explained straightforwardly.Apparently, two dissimilar materials must be present in DGH nanorod arrays, which emitted electrons under different electric field regimes.To understand the genuine mechanism for such a phenomenon, the bonding characteristics and microstructure of DGH nanorod arrays were investigated using confocal micro-Raman spectroscopy (Horiba Jobin-Yuan T64000 spectrometer; λ = 488 nm and spot size = ∼1 µm) as well as high angle annular dark field scanning transmission electron microscopy (HAADF-STEM; FEI Titan "cubed" microscope operated at 300 kV). Figure 2(e) shows the confocal micro-Raman spectra of I. patterned DGH nanorod arrays, II.NCD pads, and III.planar NCD films, which were fitted using the multi-peak Lorentzian fitting method.Spectrum III in this figure shows that the Raman spectrum of the planar NCD films contains a peak at 1334 cm 1 corresponding to sp 3 -bonded carbon (designated as "dia"), the broadened peaks at around 1352 cm 1 (D-band) corresponding to disordered carbon, and 1560 cm 1 (G-band) corresponding to graphitic phases. 38,39There also exist peaks at around 1140 cm 1 and 1520 cm 1 , which are ascribed to the ν 1 and ν 3 modes of trans-polyacetylene (t-PA) phase present in the grain boundaries. 40It is to be noted that the sharp Raman peak at 1334 cm 1 corresponding to the F2g zone center optical phonon of diamond is not clearly observable in this Raman spectrum as micro-Raman spectroscopy is overwhelmingly more sensitive to sp 2 sites.The patterning process for making NCD pads does not change the bonding structure for NCD materials profoundly such that the Raman spectrum of NCD pads [spectrum II of Fig. 2(e)] looks similar to the Raman spectrum of planar NCD films.However, RIE etching alters markedly the characteristics of the Raman spectrum of patterned DGH nanorods.Spectrum I of Fig. 2(e) shows that the "dia" and G-band become sharper, implying the improvement in crystallinity of both the nanodiamond and the nanographitic phases. 38,39A shoulder peak around 1600 cm 1 (designated as G*-band) is seen, which possibly arises from the presence of the nanocrystalline graphitic content in the films. 41Moreover, a reduction in the intensity of the t-PA peaks is observed, which is an indication of the dissociation of C-H bonds in t-PA located at the grain boundaries of DGH nanorods, pointing out that the t-PA phases were possibly converted into a more stable nanographitic phase.The Raman spectra show I D /I G values (the ratio of intensities of D-peak to G-peak) of 0.74, 0.76, and 1.05 for planar NCD films, NCD pads, and patterned DGH nanorod arrays, respectively.The increase of the I D /I G value from 0.74 to 1.05 implies the APL Mater.5, 066102 (2017) formation of nanographitic phases according to a three-stage model of increasing disorder in carbon materials, 41,42 i.e., there is conversion of sp 3 to sp 2 content in DGH nanorods due to the RIE etching process.
Figure 4(a) shows a typical cross-sectional HAADF-STEM micrograph, revealing that the DGH nanorods are intimately connected to the underlying NCD films, which are residual NCD layer not being etched away in the RIE process.The DGH nanorods are densely packed with very sharp tips.The height of DGH nanorods is very uniform, around 800-1200 nm.The inset in Fig. 4(a) shows the corresponding selected area electron diffraction (SAED) pattern, which contains sharp diffraction rings corresponding to (111), (220), and (311) lattice planes of diamond confirming the diamond nature of DGH nanorods array.There is a diffused ring that appears in the center of the SAED pattern, indicating the existence of sp 2 -bonded carbon (nanographite).A high resolution HAADF-STEM image [Fig.4(b)] shows that the DGH nanorods contain structural defects such as stacking faults and twins that is implied by the presence of the streaks in the FT 1 image of this material [inset, Fig. 4(b)].Moreover, a typical high resolution transmission electron microscopy (HRTEM; FEI Osiris at 200 kV) image taken at the tip of a DGH nanorod [Fig.4(c)] shows that there exists nano-sized diamond grains with a lattice spacing of 0.206 nm along with nanographitic phases at the grain boundaries of the diamond grains, which is confirmed by the FT 2 image [inset, Fig. 4(c)].Another typical HRTEM image [Fig.4(d)] reveals that the surface of the diamond nanorod is surrounded by graphitic phases, which is inferred by the donut-shaped central diffuse ring in the FT 3 image [inset, Fig. 4(d)].
Spatially resolved STEM-electron energy loss spectroscopy (EELS) mapping was performed in the carbon K-edge region to unambiguously distinguish between the different carbon materials such as diamond (sp 3 -bonded carbon) and graphite (sp 2 -bonded carbon) contained in the DGH nanorod. 43igures 4(e) and 4(f) show the STEM-EELS mapping with sp 3 -diamond (D, blue color) and sp 2graphite (G, pink color) for the same region depicted in Fig. 4(d), revealing that the mixture of sp 2 and sp 3 phases is present in the DGH nanorods.In Fig. 4(g), two selective area EELS spectra are plotted for the regions "I" and "II" in Fig. 4(e), which is an enlarged HAADF-STEM micrograph of a typical DGH nanorod.Both carbon K-edge spectra exhibit a typical EELS spectrum of diamond with an Fd3m structure for DGH nanorods, as they contain an abrupt rise in σ*-band (near 290 eV) and a large valley near 302 eV. 44,45Moreover, there presence a small hump near 284.5 eV (π*-band).Such an observation revealed the existence of a significant proportion of sp 2 phases at the top portion and at the grain boundaries of the DGH nanorods, which is in accord with the Raman studies of patterned DGH nanorods [spectrum I of Fig. 2(e)].The presence of sp 2 phase is possibly caused by the ion bombardment damage on sp 3 -bonded carbon induced in the RIE O 2 plasma etching process. 17,46n the basis of micro-Raman spectroscopy and HAADF-STEM investigations, the DGH nanorod is actually a nanohybrid material, consisting of nano-sized diamond grains (sp 3 -bonded carbon) and nanographites (sp 2 -bonded carbon) located in the boundaries between the diamond grains.The nanographitic phase transported the electrons upward and transferred the electrons to the tip of DGH nanorods for field emitting.Nevertheless, to understand the real mechanism for the two-slope phenomenon in the F-N plot of the patterned DGH nanorod arrays [cf.curve I of Fig. 3(b)] is complicated.Such kinds of two slopes in an F-N plot have also been observed in various other types of field emitters. 27,28,47There are numerous explanation for this two-slope F-N phenomenon, for instance, field screen effect 27,28 and space charge effect. 47However, the two-slope F-N phenomenon occurs most frequently in carbon based FEE materials, 48,49 as carbon materials are versatile in bonding structure and they are readily reacting with their environment.For example, Choi et al. 48observed the presence of adsorbates on CNTs, which resulted in two dissimilar emitting sites on the surface of CNTs, exhibiting the two-slope F-N phenomenon.Moreover, Varshney et al. 49 observed the two-slope F-N phenomenon in free standing graphene-diamond hybrid films and ascribed such a phenomenon to the simultaneous presence of graphene, diamond grain boundaries, and diamond FEE materials.
Apparently, the two-slope F-N phenomenon observed in DGH nanorods [cf.curve I, Fig. 3(b)] is also resulting from the existence of a material with a large work function (ϕ) coexisting with DGH materials.Since such a two-slope F-N phenomenon was observed only for DGH nanorods, which had undergone the RIE process for shaping the NCD pads into nanorods, it is reasonable to expect that the large-ϕ materials were formed due to the interaction of O 2 plasma with the NCD materials.Presumably, during the RIE process, the O 2 plasma bombardment first damaged the diamond materials, broke the sp 3 -bonds, and resulted in the formation of carbon dangling bonds.These carbon dangling bonds easily adsorb moisture to form a surface charge transfer layer, 50 which can also contribute to the FEE process but with large ϕ-value.Therefore, there presents line segment I b of steeper slope in F-N plots [line segment I b in curve I of Fig. 3(b)] and the DGH nanorods exhibit FEE behavior with two distinct slopes in the F-N plot.
In summary, the present study provides an inexpensive approach towards the fabrication of efficient FEE devices based on patterned DGH nanorod arrays that exhibit excellent FEE properties and high robustness.The DGH nanorods have diameters of ∼15-20 nm and heights of ∼800-1200 nm.The nanorods contain nano-sized diamond grains with nanographitic phases in their grain boundaries.Consequently, the patterned DGH nanorod arrays reveal a very low E 0 value of 5.26 V/µm, which is less than half of the corresponding E 0 value for the NCD pads.The results suggest that the "patterned seeding technique" in conjunction with the "nanodiamond masking RIE process" for fabricating the DGH nanorods is an effectual approach to improve the FEE properties of the materials.Moreover, such a process possesses large scalability and opens a new prospect for FEDs and high brightness electron sources.
FIG. 1. Schematics for the fabrication process of patterned DGH nanorod arrays: (a) chemical surface modification and seeding using ND particles, (b) spin coating of photoresist and lithography patterning, (c) plasma oxidation to etch away ND particle seeds in non-patterned areas, (d) selective area growth of NCD pads, (e) masking of the patterned NCD pads with ND particles, and (f) reactive ion etching for forming DGH nanorods in each NCD pad.

FIG. 3 .
FIG. 3. (a) The field electron emission properties (J-E curves) with (b) F-N plots for I. patterned DGH nanorod arrays, II.NCD pads, and III.planar NCD films.(c) The lifetime test, that is, the FEE current density versus time curves of patterned DGH nanorod arrays, which were tested under operation current density of 2.0 mA/cm 2 (working field of 11.0 V/µm).The inset in "a" shows the schematic of FEE measurement.

FIG. 4 .
FIG. 4. (a) Typical cross-sectional HAADF-STEM image of the DGH nanorods with corresponding SAED shown as an inset.(b) HR-STEM image of a single DGH nanorod with inset shows the corresponding FT image.(c) and (d) Typical HRTEM images at the top of the DGH nanorod with insets show the corresponding FT images.(e) HAADF-STEM micrograph and (f) composed EELS elemental mapping for diamond (blue) and graphite (pink) of a typical DGH nanorod.(g) EELS core-loss spectra taken from regions "I" and "II" designated in "e."

TABLE I .
Comparison on the field electron emission characteristics of patterned DGH nanorod arrays with other nanostructured field emitters.