Modification on the Microstructure of Ultrananocrystalline Diamond

ARTICLE pubs.acs.org/JPCC

Modification on the Microstructure of Ultrananocrystalline Diamond Films for Enhancing Their Electron Field Emission Properties via a Two-Step Microwave Plasma Enhanced Chemical Vapor Deposition Process Hsiu-Fung Cheng,† Chuang-Chi Horng,† Horng-Yi Chiang,† Huang-Chin Chen,‡ and I-Nan Lin*,‡ † ‡

Department of Physics, National Taiwan Normal University, Taipei 106, Taiwan, R. O. C. Department of Physics, Tamkang University, Tamsui 251, Taiwan, R. O. C. ABSTRACT: The electron field emission (EFE) properties of microcrystalline diamond (MCD) films were markedly improved by using ultrananocrystalline diamond (UNCD) films as the nucleation layer. Thus formed MCD/UNCD composite films possess a low turn-on field ((E0)MCD/UNCD = 10.3 V/μm), which is even smaller than that for the underlying UNCD films ((E0)UNCD = 14.7 V/μm). However, the extent of the enhancement on EFE behavior of the MCD/UNCD films is strongly influenced by the characteristics of the UNCD nucleation layer. The improvement on EFE behavior of MCD/UNCD films is large when the UNCD nucleation layer was grown in H2-free Ar plasma (CH4/Ar/H2 (0%) = 2/98/0) and is small when the UNCD layer was grown in H2-containing Ar plasma (CH4/Ar/H2 (3%) = 2/95/3). Transmission electron microscopy (TEM) examinations reveal that, while both films contain large-grain and ultrasmall grain duplex microstructure, only the former contain nanographites, locating along the interface of large-grain and ultrasmall grain regions. Presumably, the nanographites form an interconnected path that improved the transport of electrons and markedly enhanced the EFE properties of the MCD/UNCD composite films.

I. INTRODUCTION Diamond and related materials have enormous potential for applications due to their marvelous physical and chemical properties.1 3 Diamond films possess good electron field emission (EFE) properties and can potentially be applied as material for fabricating electron field emitters. There have been substantial studies carried out on the growth, properties, and applications of single crystalline and microcrystalline diamond (MCD) in the last few decades. Recently, the main focus has been directed toward the synthesis and properties of ultrananocrystalline diamond (UNCD) films,4 as the UNCD films possess many excellent properties and several of them actually exceed those of diamond.5 As the grain size in UNCD films decreases smaller than 10 nm, surface smoothness increases markedly, making it promising for device applications.6 Additionally, the decrease in diamond grain size increases the proportion of grain boundaries that contain nondiamond carbon, resulting in significant improvement in electrical properties for the films. The nondiamond contents and the crystal size of diamond grains in the films play a crucial role in the electron field emission properties of the films. A very high electron field emission characteristic has been reported for UNCD films.7 10 However, the electron conductivity of nondiamond carbons in the grain boundaries of UNCD films is not sufficiently large that limits the electron field emission properties attainable for these films. In previous papers,11,12 we developed a r 2011 American Chemical Society

modified nucleation and growth process, which utilized a layer of UNCD as nuclei and successfully synthesized a diamond film with very unique granular structure that enhanced EFE properties. However, the control of the processing parameters for obtaining such a unique microstructure is still very stringent, and how such a granular structure was formed is not clear. In this paper, how the characteristics of the UNCD layer enhanced the EFE of the MCD films was systematically investigated by changing the growth parameters for the UNCD layer. The detailed microstructure of these MCD materials grown on UNCD nuclei was examined by using transmission electron microscopy (TEM), and the mechanism that enhanced the EFE properties was proposed based on the observations.

II. EXPERIMENTAL SECTION To facilitate the optimization of plasma parameters for improving the electron field emission (EFE) properties of the composite films, the characteristics of diamond films grown directly on n-type mirror polished Si(100) substrates were examined first. Prior to the growth of diamond films, the substrates were first ultrasonicated in a solution containing diamond Received: December 21, 2010 Revised: May 30, 2011 Published: June 08, 2011 13894

dx.doi.org/10.1021/jp112131a | J. Phys. Chem. C 2011, 115, 13894–13900

The Journal of Physical Chemistry C

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UNCDI and UNCDII films, respectively. Figure 1 indicates that both UNCD films contain ultrasmall grains. The grains of UNCDII films are slightly aggregated, forming larger clusters, as compared with those of UNCDI films. Growth of the secondary diamond was carried out in the same MPE-CVD processor using H2-rich plasma (CH4/Ar/H2 = 1/49/50 sccm, 55 Torr) with 1500 W microwave power for 60 min, and thus obtained composite films were designated as MCD/UNCDI and MCD/UNCDII, respectively. The growth parameters in the MPE-CVD process are summarized in Table I to facilitate the comparison. The surface morphology of nanodiamond films was examined using a field emission scanning electron microscope (SEM, VEGA-TESCAN). The detailed microstructure of the films was examined using transmission electron microscopy (TEM, Joel 2100). The crystal quality of the nanodiamond films was investigated by Raman spectroscopy using a 514.5 nm argon laser beam (Renishaw). The EFE properties of the diamond films were measured with a tunable parallel plate setup, in which the anode, W-rod with flat end surface (about 3 mm in diameter), was controlled at about 40 μm from the cathode, the MCD/UNCD films. The current voltage (I V) characteristics were measured using an electrometer (Keithley 237) under pressure below 10 6 Torr. The sample-to-anode distance was adjusted using a digital micrometer and was monitored using an optical microscope. The EFE parameters were extracted from the obtained I V curves by using the Fowler Nordheim model.13 The turn-on field (E0) was designated as the intersection of the lines extrapolated from the low-field and high-field segments of the F N plots.

Figure 1. SEM micrograph of the UNCD nucleation layer grown in (a) CH4/Ar/H2 (0%) = 4/196/0 sccm, UNCDI, and (b) CH4/Ar/H2 (3%) = 4/190/6 sccm, UNCDII, plasma for 90 min.

powders (∼5 10 nm) for 30 min and were ultrasonically cleaned by acetone to remove any particles, which are possibly adhered on the substrates. The diamond films were grown using H2-rich plasma (CH4/Ar/H2 = 1/49/50 sccm) and a microwave plasma enhanced chemical vapor deposition (MPE-CVD) process (IPLAS, CYRANNUS-I, 2.45 GHz and 55 Torr), with the plasma excited by 1400, 1500, or 1600 W microwave power for 60 min. Thus obtained films were designated as MCDI, MCDII, and MCDIII films, respectively. Notably, no external heater was used, and the substrates were heated due to the plasma bombardment effect. The substrate temperature was measured by a thermocouple embedded in the stainless steel substrate holder and was estimated to be 850, 880, and 910 °C, respectively, during the growth of MCDI, MCDII, and MCDIII films. In the growth of composite films, the ultrananocrystalline diamond (UNCD) films were used as a nucleation layer to promote the growth of diamonds on Si substrates. The UNCD diamond nucleation layer was grown using the same MPE-CVD process, in CH4(2%)/Ar/H2 plasma (1400 W, 120 Torr) for 90 min. Figures 1(a) and 1(b) show the SEM micrographs of the UNCD seeding layers, which were grown in Ar-rich plasma (CH4/Ar/H2 = 4/196/0 sccm (0% H2) or 4/190/6 sccm (3% H2)). The substrate temperature is estimated to be 480 and 485 °C, respectively. These seeding layers were designated as

III. RESULTS AND DISCUSSION The MCD/Si diamond films grown in Ar-containing plasma (CH4/Ar/H2 = 1/49/50 sccm) possess a microstructure significantly different from that of the conventional microcrystalline diamond films grown in Ar-free plasma (CH4/Ar/H2 = 1/0/99 sccm). Contrary to the faceted grains usually contained in the conventional microcrystalline films, the MCD films grown in Arcontaining plasma consist of cauliflower-like morphology (SEM, Figure 2(a)), which contains diamond grains with irregular geometry, as illustrated by the TEM micrograph in Figure 2(b). Such a kind of granular structure is actually an aggregate of platelike grains, which intrude one another and form dendrite-like geometry.11,12 The change in plasma characteristics by applying different microwave power levels of 1400 1600 W insignificantly modifies the microstructure of the MCD films (not shown). Raman spectroscopy shown in Figure 3(a) indicates that the microwave power level also does not markedly influence the crystallinity of the MCD materials. All of the Raman spectra contain (i) D-band resonance peaks at 1320 cm 1, the characteristics of diamond materials, (ii) ν1-band (1140 cm 1) and ν2-band (1480 cm 1) resonance peaks, representing the transpolyacetylene locating at grain boundaries,14,15 and (iii) D*-band (1350 cm 1) and G-band (1580 cm 1) resonance peaks, representing the disorder carbons and graphites.16,17 However, the EFE properties for the MCD films change markedly with the microwave power level used for exciting the plasma. Figure 3(b) shows that the MCD films deposited at 1500 W microwave power exhibit better EFE properties than those grown in either 1400 or 1600 W microwave power. The turn-on field 13895

dx.doi.org/10.1021/jp112131a |J. Phys. Chem. C 2011, 115, 13894–13900


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Table I. Plasma Parameters Used for Growing the UNCD Nucleation Layer and Secondary Diamond Layer nucleation layer samples

CH4/Ar/H2 (sccm)

Ma (W)

secondary layer Pb (torr)

CH4/Ar/H2 (sccm)

MCDI

1/49/50

MCDII MCDIII UNCDI

4/196/0 (0% H2)

UNCDII

4/190/6 (3% H2)

MCD/UNCDI

4/196/0 (0% H2)

MCD/UNCDII

4/190/6 (3% H2)

1400 1400

120 120

1/49/50

Ma (W)

Pb (torr)

E0c (V/μm)

Jed (mA/cm2)

1400

55

30.0

0.08

1500

55

22.1

1.00

1600

55

34.5

0.01

1500

55

14.7

0.4

19.7

0.1

1500

55

10.3

0.8

16.7

0.05

a M: Microwave power. b P: Total pressure. c E0: Turn-on field designated as the intersection of the line segments extrapolated from the low-field and high-field segments of the F N plots. d Je: the electron field emission current density measured at 35.0 V/μm applied field (Je for MCD/UNCD films was measured at 27.0 V/μm applied field).

Figure 2. Typical microstructure of MCD films grown directly on Si substrates in CH4/Ar/H2 = 1/49/50 sccm plasma: (a) SEM morphology and (b) TEM micrograph.

Figure 3. (a) Raman spectroscopy and (b) electron field emission properties of the MCD films grown directly on Si substrates using CH4/Ar/H2 = 1/49/50 sccm plasma excited by a microwave power of 1400 1600 W.

is around (E0)1500 = 22.1 V/μm for 1500 W grown MCD films with EFE current density of 1.0 mA/cm2 at around 32 V/μm applied field. Such an EFE behavior is markedly better than the conventional microcrystalline diamond films grown in CH4/Ar/ H2 = 1/0/99 plasma with faceted granular structure, which require more than 30 V/μm applied field to turn on the EFE process (not shown).

For the purpose of improving the EFE process for MCD films, a unique two-step MPE-CVD process was adopted for growing the MCD films. UNCD films (500 nm) (UNCDI or UNCDII) were first grown on Si substrates (90 min), followed by the deposition of the MCD layer in CH4/Ar/H2 = 1/49/50 plasma for 60 min. It should be noted that the microwave power used for growing the MCD layer is 1500 W, as it leads to the best EFE 13896



Figure 4. SEM morphology of the MCD/UNCDn films synthesized on the UNCD nucleation layer, which were grown in (a) CH4/Ar/H2 (0%) = 4/196/0 sccm, UNCDI, and in (b) CH4/Ar/H2(3%) = 4/190/6 sccm, UNCDII, plasma. The MCD layer was grown in CH4/Ar/H2 = 1/49/50 sccm plasma for 60 min.

properties for MCD films (cf. Figure 3(b)). Such a two-step MPE-CVD process results in a diamond film with tremendously different microstructure from that of the MCD films directly grown on Si substrates without the UNCD seeding layer (cf. Figure 2). SEM micrographs shown in Figure 4 indicate that all of the MCD/UNCDn films possess large aggregate microstructure, containing numerous tiny particulates on their surface; i.e., it truly looks like cauliflower. Figure 5 show that the Raman spectra of MCD/UNCDn films also contain D-, ν1- and ν2-, and D*- and G-peaks, which is similar with the Raman spectra of MCD films shown in Figure 3(a), except that the intensity of resonance peaks for MCD/UNCDn films is markedly smaller than that of the MCD films. Surprisingly, the MCD/UNCDn films that were grown by the two-step MPE-CVD process exhibit markedly superior EFE properties to those of the MCD films grown directly on bare Si substrates. Figure 5(b) shows that the EFE process of MCD/ UNCDn films can be turned on at around 10.3 16.7 V/μm, as compared with the large turn-on field (22.1 V/μm) required for the MCD/Si films to initiate the EFE process. The EFE behavior of these films is strongly influenced by the characteristics of the underlying UNCD layer. The EFE process of MCD/UNCDI films can be turned on at (E0)MCD/UNCD1 = 10.3 V/μm

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Figure 5. (a) Raman spectroscopy and (b) electron field emission properties of the UNCDI and UNCDII films and the MCD/UNCDn films synthesized on the UNCD nucleation layer. The UNCDI films were grown in CH4/Ar/H2 (0%) = 4/196/0 sccm, and the UNCDII films were grown in CH4/Ar/H2 (3%) = 4/190/6 sccm plasma, whereas the MCD layer was grown in CH4/Ar/H2 = 1/49/50 sccm plasma for 60 min.

(curve III), and that of MCD/UNCDII films can be turned on at (E0)MCD/UNCD2 = 16.7 V/μm (curve IV). The EFE current density at an applied field of 24.5 V/μm achieves (Je)MCD/UNCD1 = 0.8 mA/cm2 (curve III) for MCD/UNCDI films, which is markedly larger than those for MCD/UNCDII films, (Je)MCD/ 2 UNCD2 =∼0.05 mA/cm at 24.5 V/μm applied field (curve IV). Notably, such an EFE behavior is even better than that of the UNCDI (or UNCDII) films grown in CH4/Ar/H2 (0%) = 4/196/0 sccm (or CH4/Ar/H2 (3%) = 4/190/6 sccm) plasma (curves I and II), which can be turned on at 14.7 and 19.7 V/μm, respectively. Such an observation is truly exceptional, as the UNCD films, which contain abundant grain boundaries, usually perform overwhelmingly better than the MCD films, which are of large grains and contain no interconnected path of electrons to transport. The EFE properties of these diamond films are summarized in Table I. To investigate the reason that the utilization of UNCD as the nucleation layer markedly enhances the EFE properties of MCD/UNCDn films, the detailed microstructure of the MCD/ UNCDn films was examined using transmission electron microscopy (TEM). It is interesting to observe that, contrary to the uniformly large-grain microstructure for conventional MCD films or the ultrasmall-grain microstructure for UNCD films, the MCD/UNCD films contain a duplex granular structure. There are ultrasmall grains (∼5 10 nm in size), coexisting with 13897



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Figure 6. TEM structure image of typical regions in MCD/UNCDI films, where MCD was grown in CH4/Ar/H2 = 1/49/50 sccm plasma and UNCDI nucleation layers were grown in CH4/Ar/H2(0%) = 4/196/0 sccm plasma: (a) the low magnification bright field image, (b, c) the structure image of (b) the large aggregate, (c) the small-grain region, and (d) the interfacial region of large aggregates and the small-grain region (the inset is Fouriertransformed diffractograms corresponding to the designated region).

the large grains about 50 100 nm in size (Figure 6(a)). Figure 6(b) shows a typical enlarged TEM microstructure of the large diamond grains, which were oriented along the 101 zone axis. A large number of parallel fringes appears. Detailed analysis using a Fourier-transformed diffractogram indicates that these parallel fringes correspond to either stacking fault (area 1 and FT1) or hexagonal diamond isomorphs (area 2 and FT2).18 Such a microstructure is similar with the granular structure of a conventional microcrystalline diamond grown in a plasma containing a large proportion of H2.18 In contrast, Figure 6(c) shows that, for the small-grain region (UNCD), there exists a large number of ultrasmall grains (around 5 8 nm) separated by amorphous grain boundaries of the thickness around 1 2 nm, which is very similar to the typical granular structure of UNCD films grown in H2-free Ar/CH4 plasma.12 Restated, the MCD/UNCDI films contain large diamond grains distributed among the UNCD matrix. The large diamond grain regions contribute relatively little for the enhanced EFE process, as they contain very few grain

boundary phases and are basically insulating. Presumably, the marvelous EFE behavior for MCD/UNCDI films results from the presence of UNCD regions, which bypass the electron transport and markedly improve the EFE process. Notably, the EFE of MCD/UNCDI films is even better than that of UNCDI films grown in CH4/Ar/H2 (0%) = 4/196/0 sccm plasma, i.e., (E0)MCD/UNCD1 = 10.3 V/μm and (E0)UNCD1 = 14.7 V/μm. Therefore, there must be another factor that results in better EFE properties than the presence of amorphous carbons along the grain boundaries in UNCD films. To investigate the possible source of enhancement, the interface regions between the large diamond grains and the small UNCD grains were examined. It should be noted that to examine the interface regions the samples were tilted such that the large diamond grains are well off the zone axis and insignificantly diffract the electrons. Figure 6(d) clearly indicates that there exists a graphitic phase located at the interface region between the large grains and UNCD grains (areas 3 and 4, FT3 and FT4). It is 13898



Figure 7. TEM images of typical regions in MCD/UNCDII films, where MCD was grown in CH4/Ar/H2 = 1/49/50 sccm plasma and the UNCDII nucleation layer was grown in CH4/Ar/H2(3%) = 4/190/6 sccm plasma: (a) the bright field image, (b) enlarged bright field image of a typical large aggregate, and (c) the structure images of the large aggregate region and the associated Fourier-transformed diffractograms.

believed that the nanographites existing in these interface regions form interconnected paths, which have better conductivity than the amorphous phase located at grain boundaries of UNCDI, and largely enhance the EFE properties for MCD/UNCDI films. It should be noted that the EFE properties of the MCD/UNCD films can not be correlated directly with the through-film resistance.

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The through-film resistance was estimated by measuring the I V characteristics of the MCD/UNCD films using a four-probe configuration, with two of the probes in contact with the underlying Si substrates and the other two in contact with the top surface of MCD/UNCD films. The resistance from the Si to MCD/UNCD films, crossing the Si-to-diamond interface, can thus be evaluated. The through-film resistance is extremely large (∼800 MΩ) when the driving current is around 100 nA. For small voltage applied crossing the MCD/UNCD/Si layer, the MCD/UNCD is in the intrinsic conduction regime, and the electrons do not conduct very efficiently. The significance of such an observation is that, before the turn on of the electron field emission process, the electrons must be transported via some tunneling process, such as hopping between the graphitic islands existing in the grain boundaries. Moreover, the characteristics of the underlying UNCD layer also impose significantly the extent of enhancement that the secondary MPE-CVD process for growing the MCD layer improved the EFE properties of the MCD/UNCD layer. Figure 5 shows that the MCD/UNCDII films shows EFE properties of (E0)MCD/UNCD2 = 16.7 V/μm, which are still inferior to those of UNCDI films ((E0)UNCD1 = 14.7 V/μm). Apparently, the ineffectiveness in promoting the EFE behavior of composite films by using UNCDII as the nucleating layer is the failure in forming the nanographites between the interfaces. The TEM micrograph shown in Figure 7(a) indicates that there also exist abundant large aggregates (∼200 nm) distributed among the small-grain matrix in MCD/UNCDII films. The detailed microstructure shown in Figure 7(b) illustrates that the large aggregates possess very complicated microstructure. The structure image for one of the typical regions in the large aggregates is shown in Figure 7(c) to reveal that they are also diamond, containing stacking faults and a hexagonal polymorph. It should be noted that the micrographs in Figure 7(b) were deliberately defocused so as to delineate the granular structure of these regions. It indicates that the regions adjacent to the large aggregates contain grains of the size around ∼40 60 nm that were oriented in a nondiffracting direction. The size of these small grains is nearly one order of magnitude larger than that of the ultrasmall grains usually observed in typical UNCD films (cf. Figure 6(c)), indicating that the grain growth process has occurred for the ultrasmall grain region of MCD/UNCDII films. However, there is no nanographite phase observable near the interface between the large aggregates and the adjacent region. It should be noted that the UNCDI films grown in CH4/Ar/ H2 (0%) = 4/196/0 sccm plasma contain ultrasmall spherical diamond grains (∼5 nm) separated by grain boundaries, which contain amorphous carbons (Figure 8(a) and inset). In contrast, Figure 8(b) shows that in the UNCDII films grown in CH4/Ar/ H2 (3%) = 4/190/6 sccm plasma, which contain 3% H2 in the CH4/Ar plasma, the amorphous carbon in grain boundaries was completely eliminated and that the anisotropic growth of diamond grains was initiated, resulting in dendrite-geometric grains about 200 nm in size, which are actually platelike diamond grains.12 The films became very resistive. It was expected that the secondary CVD process for growing the MCD layer still induces the coalescence of the nanosized diamond grains to result in the formation of large aggregates. However, no nanographite phase was formed, and the EFE properties of these films can not be further enhanced beyond the UNCDII films. These observations infer that the presence of amorphous carbons in the 13899



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than the turn-on field for UNCD films ((E0)MCD/UNCD =10.3 V/μm and (E0)UNCD =14.7 V/μm), and attain an EFE current density, which is even larger than the Je for UNCD films ((Je)MCD/UNCD = 0.8 mA/cm2 and (Je)UNCD = 0.4 mA/cm2 at an applied field of 35.0 V/μm).

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank the National Science Council, Republic of China, for the support of this research through the Project No. NSC 99-2119-M-032-003-MY2. ’ REFERENCES

Figure 8. TEM micrographs of the (a) UNCDI and (b) UNCDII nucleation layer, which were grown in CH4/Ar/H2 (0%) = 4/196/0 sccm and CH4/Ar/H2 (3%) = 4/190/6 sccm, respectively, for 90 min (the inset in (a) is the structure image of a typical region in UNCDI).

UNCD nucleation layer is of crucial importance to induce the formation of nanographites at interface regions, to enhance the EFE properties of the composite diamond films.

IV. CONCLUSION The EFE properties of MCD films grown in CH4/Ar/H2 = 1/49/50 sccm plasma were markedly improved by using UNCD films as the nucleation layer. TEM examinations reveal that it forms composite films that contain large diamond grains distributed among the small UNCD grains. The UNCD phase still predominates the EFE characteristics of the composites. However, the enhancement of EFE behavior of the MCD/UNCDn composite films is large only when the UNCD nucleation layer contains amorphous carbon (a-C) in the grain boundaries (grown in CH4/Ar/H2 (0%) = 4/196/0 sccm plasma). The improvement on EFE properties of the MCD/UNCD films is small when the UNCD nucleation layers are grown in CH4/Ar/ H2 (3%) = 4/190/6 sccm plasma and contain no a-C phase grain boundaries. Presumably, the enhanced EFE properties are owing to the presence of nanographites along the interface of large MCD grains and UNCD grains, which form an interconnected path and facilitate the transport of electrons. The resulting MCD/UNCD films possess a turn-on field, which is even smaller

(1) Chakrabarti, K.; Chakrabarrti, R.; Chattopadhyay, K. K.; Chaudhrui, S; Pal, A. K. Diamond Relat. Mater. 1998, 7, 845. (2) Ralchenko, V.; Karabutov, A.; Vlasov, I.; Frolov, V.; Konov, V.; Gordeev, S.; Zhukov, S.; Dementjev, A. Diamond Relat. Mater. 1996, 8, 1496. (3) Wang, S. G.; Zhan, Q; Yoon, S. F.; Ahn, J.; Wang, Q.; Zhou, Q; Yang, D. J. Phys. Status Solidi A 2002, 193 (3), 546. (4) Gruen, D. M. Annu. Rev. Mater. Sci. 1999, 29, 211. (5) Carlisle, J. A.; Auciello, O. Electrochem. Soc. Interface 2003, 28. (6) Mortet, V.; Elmazria, O.; Nesladek, M.; Assouar, M. B.; Vanhoyland, G.; D’Haen, J.; Olieslaeger, M. D.; Alnot, P. Appl. Phys. Lett. 2002, 81, 1720. (7) Zhu, W.; Kochanski, G. P.; Jin, S. Science 1998, 282, 1471. (8) Lee, Y. C.; Lin, S. J.; Chia, C. T.; Cheng, H. F.; Lin, I. N. Diamond Relat. Mater. 2004, 13, 2100. (9) Pradhan, D.; Lee, Y. C.; Pao, C. W.; Pong, W. F.; Lin, I. N. Diamond Relat. Mater. 2006, 15, 2001. (10) Lee, Y. C.; Lin, S. J.; Pradhan, D.; Lin, I. N. Diamond Relat. Mater. 2006, 15, 353. (11) Wang, C. S.; Chen, H. C.; Cheng, H. F.; Lin, I. N. Diamond Relat. Mater. 2009, 18, 136. (12) Wang, C. S.; Chen, H. C.; Cheng, H. F.; Lin, I. N. J. Appl. Phys. 2010, 107, 034304. (13) Fowler, R. H.; Nordheim, L. Proc. R. Soc. A 1928, 119, 173. (14) Sun, Z.; Shi, J. R.; Tay, B. K.; Lau, S. P. Diamond Relat. Mater. 2000, 9, 1979. (15) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2001, 63, 121405. (16) Michler, J.; Von Kaenel, Y.; Stiegler, J.; Blank, E J. Appl. Phys. 1998, 81 (1), 187. (17) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095. (18) Lin, I. N.; Chen, H. C.; Wang, C. S.; Lee, Y. R.; Pong, W. F.; Lee, C. Y., CrystEngComm in press (2011).

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Modification on the Microstructure of Ultrananocrystalline Diamond

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