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, 2004, 46, . 4 Electron emission properties of detonation nanodiamonds,, V.V. Zhirnov, O.A. Shenderova, D.L. Jaeger, T. Tyler, D.A. Areshkin, D.W. Brenner, J.J. Hren North Carolina State University, Raleigh, NC 27695-7907, USA International Technology Center, Research Triangle Park, NC 27709, USA This paper summarizes results of systematic studies of field electron emission from detonation nanodiamond coatings corresponding to nanodiamond powders of different modifications. The role of chemical composition of the surface of detonation nanodiamond particles in field emission mechanisms is discussed. Field emission related electronic properties of single diamond nanodots are studied using tight-binding calculations and continuum electrostatic simulations.

D.W.B., O.A.S. and D.A.A. acknowledge funding by the Office of Naval Research (Contract N 00014-95-1-0279), O.A.S. and D.L.J. the Office of Naval Research (Contract N 00014-02-1-0711).

1. Introduction 2. Detonation Nanodiamond Coatings Extensive studies of field electron emission from diamond Detonation DND were deposited onto sharp field emishave been carried out during the past several years (for sion tips (Si and Mo; curvature radius 10-100 nm) a review see ref. [1]). It was observed experimentally by pulsed electrophoresis, in a suspension of diamond that coating metal field emitters with diamond films nanoparticles in alcohol (for details see Refs. [24]). The (CVD, natural diamond, HPHT, etc.) could significantly resulting deposits of diamond nanoparticles were found enhance electron emission [1]. However, the mechanism to depend upon applied voltage, suspension concentration, of electron emission in such structures is not completely pulse duration, and tip geometry. This procedure resulted understood because they are complex. The properties of in sufficient control to prepare metal needles with deposits the surface (e. g. electron affinity), new interfaces (e. g. grain varying from isolated diamond nanodots to continuous boundaries), material inhomogeneities, and/or doping can nanodiamond films of varying thickness.

have dramatic effects on the emission behavior of these Three different types of detonation diamond [5], shown in composite emitters. Table 1, were investigated. Detonation diamond nanoparticles were prepared in the Russian Federal Nuclear Center Diamond nanodots (DND) produced by detonation are All-Russian Institute of Technical Physics (VNIITF the smallest (2-5 nm in size) currently observed particles Snezhinsk).

of diamond matter, with many properties still unknown.

Emission J-F (current density vs electric field) characteDeposition of the detonation DND on the top of field emission tips allows one to obtain information about elec- ristics were measured. To compare different coatings of different type of nanodiamond, the two parameters tronic properties of DND from field emission experiments were chosen: emission threshold field (Fth), and integral along with information about structure and composition 1 J normalized transconductance (gn = ), which as is a from non-destructive high-resolution transmission electron J F measure of steepness of J-F characteristics. As can be microscopy.

Table 1. Properties of three types of detonation nanodiamond particles [5] and field electron emission parameters of corresponding nanodiamond coatings Nanodiamond type USDD2 USDD3 USDDAlternative name Nd NdP1 NdO Description standard USDD2 with additional USDD3 with additional acid & high-temperature treatment ozone treatment Impurities O,N,H 8-10% O,N,H 2-4% N,O 6% pH of 10% water susp. 5.66.2 3.54.5 1.62.Field electron emission parameters Change in field emission threshold, -15% -25% -15% compared to bare Si field emitter Change in normalized integral transconductance, -37% -30% +33% compared to bare Si field emitter 5 642 V.V. Zhirnov, O.A. Shenderova, D.L. Jaeger, T. Tyler, D.A. Areshkin, D.W. Brenner, J.J. Hren ver, the high-current part in the emission characteristics shifted up, indicating an increase in transconductance.

The difference in emission properties of NdO and NdPnanodiamond coatings reflects the importance of surface modification. The emission results can be interpreted in terms of the two-barrier MetalDiamondVacuum emission model. In this model, electrons are injected from the conductive electrode (e. g. field emission tip) into diamond through an interface barrier. Then the electrons move to the diamond-vacuum interface, and escape to vacuum through the surface barrier (e. g. the electron affinity).

The transparency of both barriers determines the emission threshold. During electron transport in diamond, there are several physical phenomena resulting in the resistance of the diamond film, such as negative and positive space charge effects, scattering, hopping conductance, etc. These resistive effects limit the supply of electrons to the surface, which result in shallow or saturated current-voltage characteristics in the higher current region (e. g. lower transconductance).

Nanodiamond of NdO type was treated by ozone, and correspondingly, it contains larger amounts of oxygen on the surface. Oxygen, being an electronegative element, is known to increase electron affinity of diamond surface.

Hydrogen plasma treatment replaces oxygen with hydrogen, which is known to decrease the electron affinity of diamond.

Based on the FowlerNordheim equation for field emission, the shift of emission characteristics in Fig. 1, a corresponds Figure 1. Hydrogen plasma effect on emission characteristics of to a decrease in the effective surface barrier of 0.9 eV.

detonation nanodiamond coatings: (a) NdO and (b) NdP1.

Nanodiamond of NdP1 type (without ozone treatment) apparently contains smaller amounts of oxygen, but larger amounts of H and N atoms on the surface. Its emission threshold before H-plasma treatment is lower than NdO, seen from Table 1, all three types of nanodiamond coatings however the transcondactance is lower, due to resistive showed an improvement in the emission threshold, the (e. g. negative shace charge) effects at higher currents. After smallest threshold was observed for NdP1 coating. HoweH-plasma, the emission threshold did not change, however, ver, the effect of the coatings on the normalized integral the resistance effects became smaller (the transconductransconductance (steepness) was different for different tance increased). This could indicate on a role of nitrogen coatings. While the NdO coating resulted in much steeper in the resistance effects, since oxygen-rich samples did not J-F characteristics, as compared to the bare Si emitter, show the resistance effects, and the hydrogen content after emitters with detonation nanodiamond and NdP1 coatings plasma treatment at the same conditions should be similar showed very shallow characteristics in the higher current in both samples.

region (low transconductance). Additional hydrogen plasma treatment was used to modify surface properties of emitters with NdO and NdP1 coatings (for details see ref. [2]). Fig. 3. Field emission from a single shows emission characteristics before and after the plasma nanodiamond particle treatment. As can be seen, the effects of hydrogen plasma are very different for the two types of nanodiamond.

We attempted to reduce the possible variables by perFor the NdO coating, emission threshold remarkably forming experiments with controlled deposits of nanodiadecreased after H-plasma, and J-F characteristic shifted mond particles (USDD4 NdO type) with an individual to the left. The emission threshold improvement was 35% size of about 5 nm. Both the geometry of the underlying relative to the NdO emitter before hydrogen plasma treatmetal surface and the diamond particle were observed ment and 45% relative to the bare Si field emitter. The with atomic resolution in a transmission electron microhydrogenated NdO coating was found to be the best scope. In the experiments reported here, isolated diamond emissive coating in this series of experiments, with both nanoparticles were deposited onto Mo tips (Fig. 2, a), lowest emission threshold and highest transconductance.

characterized by field emission, and then the resulting data For the NdP1 coating (Fig. 1, b), the emission threshold compared to that of the bare metal tip before deposition did not change after treatment in hydrogen plasma. Howe- (of the same specimen). Next, thicker nanodiamond coating , 2004, 46, . Electron emission properties of detonation nanodiamonds Table 2. Geometrical and emission parameters of nanodiamond field emitters Characteristics Bare Mo Single particle ND film Metal tip radius 50 nm 50 nm 50 nm Geometrical ND Thickness 0nm 2.3 nm 20 nm Threshold voltage 172 V 222 V 89 V Field Emission Transconductance 7.63 nA/V 7.09 nA/V 16.4 nA/V FowlerNordheim Analysis Apparent work function 4.05 eV 5.57 eV 2.71 eV was deposited onto the same tip with subsequent field 4. Simulation of field emission related emission measurements. The results of these experiments properties are summarized in Table 2.

Comparing the IV curves of the bare Mo tip with the As part of our efforts to develop a reasonable quantitative same tip with a deposit of one isolated diamond nanoparticle model of electron emission from diamond nanoparticles, yields a substantial increase ( 30%) in threshold voltage the electronic structure of diamond nanodots containing (Fig. 3, b and Table 2). However, after the additional between 34 and 1600 carbon atoms was calculated using deposition of nanodiamond and the formation of a thicker a self-consisting tight-binding Hamilonian [6,7]. Analysis nanodiamond film, a drastic decrease in threshold voltage of these results indicate that for cluster sizes larger than ( 48%) and an increase in transconductance ( 115%) approximately 2 nm quantum confinement effects have relaare obvious (Fig. 3, b and Table 2). tively little effect, and the electrostatic potential distribution is essentially insensitive to cluster size for clusters larger than 1 nm. The calculated electron affinity (EA) of hydrogenated diamond dots with a majority of their surface area corresponding to (111) facets was approximately 1.4 eV. This result is close to the experimental value of -1.27 eV [8] and the theoretical value of -2.0 eV calculated using first principals density functional theory by Robertson and Rutter [9] for a bulk hydrogenated (111) diamond surface. In the present paper we address the effect of size on EA in more detail.

It had been demonstrated that the electron affinity of diamond depends on the polar groups on the surface [8].

The sign and magnitude of the surface dipole layer induced potential drop due to specific polar groups enhances and uniquely determines the electron affinity (Fig. 3, a).

Fig. 3, b illustrates continuum electron electrostatic potential distributions within a dielectric sphere with two oppositely charged layers on the surface. The distance between charged layers (0.5 ) and the related charge density (0.145 e per carbon atom) corresponds to the CH dipole strength on a (111) diamond surface obtained from the tight-binding simulations. The positive potential drop within the particle shifts the electron energy spectrum up, and hence decreases the EA. Switching the polarity of the charged layers results in an increase in the EA.

For qualitative understanding of the possible effect of size on EA, we assume that introducing a curvature to the initially flat dipole layer corresponding to the macroscopic surface will result in a larger separation of the positively charged H centers for the outer layer (Fig. 3, a), and therefore inferior conditions for emission. We also assume Figure 2. A single, isolated tightly bound, detonation diamond that the surface area per carbon atom for nanodiamond is nanodot on the tip of a needle (a). Field emission characteristics similar to that of macroscopic surfaces. Recently, it has from the isolated nanodiamond particle shown in Fig. 2, a and a been demonstrated that atomic geometrical parameters of nanodiamond film, compared to a bare tip (b). Inset: schematic of experimental setup. hydrogen terminated nanodiamond do not differ appreciably 5 , 2004, 46, . 644 V.V. Zhirnov, O.A. Shenderova, D.L. Jaeger, T. Tyler, D.A. Areshkin, D.W. Brenner, J.J. Hren Figure 3. Illustration of size effect of electron affinity properties of nanodiamond particle. Dipole induced negative electron affinity for hydrogenated diamond surface (a). Dipoles are shown for macroscopic flat and curved nanoparticle surfaces. Electrostatic potential profile along a radius of a nanodiamond particle calculated for a dielectric sphere with two oppositely charged layers with uniformly distributed charge (b) and with pointed charges on the surfaces (c). Coulomb potential profile for two diamond clusters calculated with self-consistent tight-binding Hamiltonian (d).

from those for bulk diamond [10]. Given the assumptions above the effect of size on the potential drop can be evaluated through the equation for the potential distribution inside a spherical capacitor q d R U =.

Aa R + d Here q and d correspond to the charge and length of a dipole, A is surface area per carbon atom (5.4 2), R is the radius of the inner sphere (carbon atoms), and a is the absolute dielectric permittivity. Fig. 4 illustrates the dependence of the voltage drop on the size of the particle using continuum electrostatic simulations. An appreciable effect takes place only for particles with radii less than 1 nm. Self-consistent tight-binding simulations indicate Figure 4. Illustration of the size dependence of the potential a less pronounced size effect for the simulated particles drop within a nanodiamond particle, changing the electron affinity.

having a shape of truncated octahedrons. A slight decrease Dipole characteristics are d = 0.5, q = 0.14e; area per carbon atom: A = 5.4 2.

( 0.1V) of the potential can be seen only for a particle , 2004, 46, . Electron emission properties of detonation nanodiamonds less than 1 nm in diameter (Fig. 3, d). This can be attributed to the fact that simulated particles are faceted and their surfaces are flat rather than spherical as is assumed in the continuum electrostatic calculations above. Therefore based on the present results, the effect of size on the EA of a nanoparticle is sensitive to the shape of the particle.

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