CurrentâVoltage Characteristics of in Situ Graphitization of

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Current-Voltage Characteristics of in Situ Graphitization of Hydrocarbon Coated on ZnSe Nanowire Y. G. Wang,†,‡ M. X. Xia,†,‡ B. S. Zou,† T. H. Wang,† W. Han,§ and S. X. Zhou*,†,§ State Key Laboratory for Chemo/Biosensing and Chemometrics, and Key Laboratory for Micro-Nano Optoelectronic DeVices of Ministry of Education, Hunan UniVersity, Changsha 410082, China, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China, and National Amorphous and Nanocrystalline Alloy Engineering Research Center, Central Iron and Steel Research Institute, XueYuan Nan Road No. 76, Beijing 100081, China ReceiVed: December 24, 2009; ReVised Manuscript ReceiVed: June 16, 2010

Current-voltage characteristics of hydrocarbon coated on ZnSe nanowire annealed by current-induced Joule heating have been studied via an in situ transmission electron microscope (TEM) technique. The major feature of the measured current-voltage curves is the presence of negative conductance due to a remarkable drop of current at the early stage of the graphitization which decreases the density of the band tail states resulting from hydrogen that escaped from various C-Hn clusters and the sp3 and sp2 coordinated carbon networks. At the later stage of graphitization, the current began to increase rapidly due to sp3 coordination transformation into sp2 coordination, which effectively narrowed the band gap. Finally, a linear relationship between the current and voltage appeared as a result of a phase transition from the amorphous hydrocarbon to graphite, leading to the semiconductor to conductor transition. The details revealed in the measured current-voltage curve strongly suggest that the electrical property of the hydrocarbon deposition may be optimized by intentional arrangement with current-induced Joule heating. 1. Introduction Amorphous carbon, including hydrocarbon, have some advantages including a low background current, a large potential window, and inertness in chemically aggressive environments. It may contain a fraction of sp3 carbon bonds stabilized by hydrogen in a wide range from near zero to 100%, which results in a varied microstructure from diamond-like to graphite-like configurations depending on the deposition conditions and hydrogen content. Consequently, the electrical property of the amorphous carbon can vary between insulator and conductor, allowing the amorphous carbon to be applied in field emission,1,2 microelectronic devices,3-5 hard coatings,6,7 and gas sensors.8,9 Amorphous carbon films are also good candidates for novel electrode materials in microfluid biodetection chips for electrochemical analysis because they have a low dielectric constant and excellent gap-filling capabilities.10-12 Each of these applications requires that the properties of the amorphous carbon are optimized to fulfill different functionalities, including hardness, surface smoothness, optical transparency, electrical conductivity, field emission, etc., which usually involve only the microstructure and volume fraction of the sp3 bonds. On other hand, hydrocarbons are also important precursors and solders for construction of complex nanostructures such as bent and zigzag carbon nanotube structures and more complex networks consisting of crossed and T-junctions, coatings on nanowires, nanowires grown on carbon nanotubes, and Ti emitter tips via in situ deposition under an electron microscope.13-19 These nanostructures can be fabricated accurately at a preselected position on the nanomaterials and have possible techno* To whom all correspondence should be addressed. E-mail: sxzhou@ atmcn.com. † Hunan University. ‡ Chinese Academy of Sciences. § Central Iron and Steel Research Institute.

logical applications in the ongoing miniaturization of optoelectronics and sensing devices. The hydrogenated amorphous carbon fabricated by electron beam-induced deposition is a semiconductor with a band gap larger than 1.5 eV,20,21 which is suitable for application in solar cells. It is generally believed that the microstructure plays a key role in determining the properties of hydrocarbon.17,22 Postannealing can modify the microstructure of the hydrogenated amorphous carbon and hydrocarbon and thus has been used to adjust important electrical and mechanical properties for potential applications in electrically driven nanodevices. A close relationship between the microstructure and the transport properties of the hydrogenated amorphous carbon allows the band gap to be controlled to some extent by modification of the microstructure via posttreatment.23 The tunable property of band gap is also attractive for the application of hydrogenated amorphous carbon and/or amorphous hydrocarbon to photovoltaic solar cells. In order to use these amorphous carbon films for the fabrication of high efficiency multiband gap solar cells, it is necessary to control the optical band gap of these amorphous carbon films. Postannealing can also modify the surface adhesion capability of the tetrahedral amorphous hydrogenated carbon films, leading to enhanced blood compatibility.24 In order to optimize the physical properties of the hydrocarbon nanostructures via posttreatment, it is necessary to carefully design the parameters of annealing according to requirements of a specific application. Consequently, the information derived from current-voltage characteristics describing the electrical property at different stages of graphitization is of importance for proper design of the posttreatment. Although there have been some experimental studies about the posttreatment of amorphous carbon and hydrocarbon,17,25-31 to our knowledge, there is still a lack of information about direct measurement of current-voltage characteristics of hydrocarbon during annealing, especially the

10.1021/jp103466x  2010 American Chemical Society Published on Web 07/12/2010

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details of conductance at different stages. The main objectives of this study are to investigate current-voltage characteristics of amorphous hydrocarbon during current-induced Joule heating and to examine conductance alternation to aid optimization of the electrical property of amorphous hydrocarbon via postannealing. We reveal that the current initially decreased significantly, possibly due to hydrogen that escaped from the sp3 positions which can effectively reduce the density of band tail states and increase the number of the dangling bonds. Then the current increased rapidly, possibly due to sp3 bonds that transformed into sp2 bonds, which effectively narrowed the band gap. The experimental current-voltage results may facilitate the selection of annealing parameters to optimize the electrical property of the amorphous hydrocarbon nanostructures.

Wang et al. system (Gatan, model 691), the hydrocarbon-coated nanowire was probed at nanoscale accuracy in three-dimensional spaces under TEM inspection. The Nanofactory SU100 controller, with bias voltage ranging from -10 V to 10 V, was used as a direct current voltage source for annealing the hydrocarbon coating by current-induced Joule heating in order to modify the microstructure and the electrical property of the as-fabricated hydrocarbon. In situ measurement of the current with picoampere sensitivity was performed. A total of 1000 current values were measured for the applied bias voltage range, and the interval of the applied bias was equal to 1/1000 of the real bias voltage range. To obtain more information linked to the conductance variation during the heating process, 50 µs was used for recording a single current value. The time interval between two successive measurements of the current was also set to 50 µs.

2. Experimental Section ZnSe nanowires with an average diameter of about 20 nm and an axial axis along the [111] direction were prepared on the [111]-oriented GaP wafer by a metal-catalyzed vaporliquid-solid (VLS) process and transported to a fixed ground plate counter-electrode made of gold film with a thickness of about 50 µm and width of 1 mm by making a smooth scratch with the ZnSe nanowires on the GaP substrates using this gold electrode, which resulted in alot of free-standing ZnSe nanowires protruding from the electrode to be used as a template.32 Then the electrode with the ZnSe nanowires was mounted onto the piezodriven manipulation TEM-STM holder (Nanofactory Instruments, model HS 100STM-holder). After this specially designed sample holder was inserted into the TEM, the electrode with the ZnSe nanowires was delicately positioned inside a pole piece gap of the TEM by means of a horizontal shift operation in the X and Y directions. A conventional TEM (model JEOL2010) operated at 200 kV was used to produce the hydrocarbon coating on the nanowire template by carefully focusing the incident electron beam on the electrode. A small amount of organic molecules (mainly hydrocarbons) are always present in the TEM column due to contamination from pump oil (hydrocarbon-based oil) vapor in the TEM vacuum system,15-17 which can be used as the gas precursor for the hydrocarbon coating. Under electron beam irradiation, the organic molecules (mainly hydrocarbons with different straight, radial, and ring configurations) are polarized or ionized. Attracted by the electrical field around the charged nanowire, the polarized or ionized organic molecules can adsorb and aggregate at the surface of the nanowire to form the coating layer. As a result of complex electron beam-induced surface reactions such as polymerization, the adsorbed organic molecules and hydrocarbons dissociate into deposited amorphous carbon and hydrocarbon mixtures, and volatile fragments are pumped out by the vacuum system of the TEM. Principally, the nature of the obtained hydrocarbon coating can be regarded as hydrogenated amorphous carbon. The deposition lasted for about 30 min, and a large angle rotation (approximately (30°) of the holder was carried out clockwise and anticlockwise, respectively, during the deposition in order to deposit the hydrocarbon on the surface of the nanowire as uniformly as possible. The microstructure of the hydrocarbon coating was similar to that of the amorphous hydrocarbon. The presence of hydrogen in the deposited hydrocarbon was confirmed by electron energy-loss spectrometry (EELS). The current-induced Joule heating and electrical measurements were also carried out in situ inside the TEM. By moving the piezodriven probe electrode with a sharp tip made by Ar ion bombardment at a glancing angle of 5° and accelerating voltage of 4 kV using a precision ion polishing

3. Results and Discussion 3.1. Current-Voltage Characteristics of Graphitized Hydrocarbons. Figure 1a is a TEM image showing the electrical circuit consisting of the as-deposited hydrocarbon coated on the ZnSe nanowire with a diameter of about 16 nm and two electrodes to be used for thermal annealing of the hydrocarbon, where the ground plate counter-electrode only shows a very narrow rim at the left edge where the nanowires are standing. The microstructure revealed by the inset in the left top corner corresponds to the amorphous carbon. The EELS spectra obtained at image mode reveal the presence of hydrogen in the coating, as shown in Figure 1b and 1c. Generally, there are two peaks at about 27 and 6.4 eV in graphite, produced by the energy loss of transmitted electrons to the plasmon oscillation of the valence electrons and named (π+σ) and π peaks, respectively.33,34,42,43 In amorphous carbon the loss of long-range order led to a relaxation of the selection rules for transitions and a smearing of the energy levels. As a result, the π peak strongly decreased in intensity and became broader, and its intensity was used to qualitatively determine the sp2 concentration in the amorphous carbon. The π and (π+σ) plasmon peaks shifted to a lower energy loss of about 4-6 and 23.5 eV, possibly due to a reduction in density of the hydrogenated amorphous carbon in comparison to graphitized carbon.35,36 The peak positions and the features of the EELS in Figure 1b agree with the characteristics reported for the amorphous carbon or/ and hydrogenated amorphous carbon except for the prominent shoulder-like peak at about 13 eV, which results from the ionization of hydrogen and definitely indicates the presence of hydrogen in the amorphous carbon coating. Usually, the peak resulting from k shell ionization of hydrogen is very weak, although a large amount of hydrogen is present in the hydrogenated amorphous carbon.33,34 The EELS spectrum associated with the inner-shell ionization is shown in Figure 1c. The sharp peak observed at above 290 eV originates from the electron transition from 1s state to σ* state and indicates a high density of sp3-bonded atomic sites in the hydrogenated amorphous carbon,35 while a shoulder-like peak at 285.5 eV produced by 1s to π* transitions indicates a low density of sp2-bonded atomic sites in the hydrocarbon coating.33,34 Therefore, the deposited hydrocarbon could be considered as hydrogenated amorphous carbon. On the basis of the reported mass density of hydrogenated amorphous carbon (1.6-2.1 g/cm3),33,34 the hydrocarbon mass per nanometer on the ZnSe nanowire was estimated to be 5.3-6.9 × 10-19 g. Because hydrogen can fit into virtually any space in the carbon materials including on lone dangling bonds and results in a lower formation energy of the carbon network by reacting with the dangling bonds, it provides significant

Graphitization of Hydrocarbon Coated on ZnSe Nanowire

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Figure 1. (a) TEM image shows the as-synthesized hydrocarbon coated on ZnSe nanowires that are squeezed between two gold electrodes for in situ Joule heating. The inset clearly shows the microstructure of the hydrocarbon coating and the amorphous nature of the as-prepared hydrocarbon. Low energy (b) and high energy (c) EELS spectra of the as-prepared hydrocarbon coating.

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Figure 2. (a) TEM image and (b) low energy EELS spectrum of the graphitized hydrocarbon after in situ annealing. The inset reveals the misoriented layer structure in the resultant carbon nanotube.

stabilization of the amorphous structure of carbon. The hydrogen mainly bonded to sp3 sites, but an amount of hydrogen bonded to sp2 sites in olefinic and aromatic systems. Because hydrogen is singly bonded, it can act only as a network terminator, thus reducing the amount of cross-linking and network constraints. Therefore, at higher hydrogen concentrations, it is possible to have a higher fraction of tetrahedrally coordinated carbon without becoming constrained. Hydrogen is favorable for formation of sp3 bonds, and thus such amorphous carbon often contains an amount of hydrogen, depending on the preparation techniques and conditions, usually ranging from 20 at.% to 40 at.%, but possibly as low as 10 at.% and high as 60 at.% under special deposition conditions.33,34,37 On the other hand, a decreasing amount of hydrogen may give rise to an increase in the concentration of free bonding orbitals due to increasing number of the unpaired dangling bonds at sp3 and sp2 sites, respectively. Because hydrogen mainly bonds to sp3 sites, the hydrogen content has a close relationship to the sp3/sp2 ratio.

After in situ Joule heating, the carbon nanostructure was formed and part of the ZnSe nanowire was removed as a result of thermal evaporation as shown in Figure 2a. The probe electrode was moved away in order to reveal the whole resultant carbon sheath. Because the sp3 sites start to convert into sp2 sites under vacuum conditions when temperature is elevated to about 1400 °C, while ZnSe began to melt at about 1500 °C and evaporate before fully molten, the ZnSe nanowires were not completely removed by thermal evaporation during the graphitization process and some remnants were found inside the resultant carbon nanotubes with a thickness of about 3 nm. Because graphitized carbon nanotubes are a conductor by nature, the residual semiconductor ZnSe inside the carbon nanotube has no effect on electron transport. The inset at the middle of the right edge of Figure 2a shows a layer structure in the resultant carbon nanotube. Misorientation between these layers is clearly observed to be in a range less than 30° as indicated. The spacing of the neighboring layers is about 0.34 nm,


Figure 3. (a) The current-voltage curve recorded during in situ annealing of hydrocarbon on the ZnSe nanowire. The negative conductance occurred at the position indicated by letter S. The linear relationship between current and voltage is indicated by letter F. (b) Current-voltage plot recorded for the graphitized carbon nanotube shows an ohmic characteristic between the voltage and current.

coincident with the (0002) lattice planar spacing of the graphite. Figure 2b is the EELS spectrum of the resultant carbon nanotube, revealing an obvious variation of the characteristic features of the peaks. The prominent π peak at 6.4 eV suggests that the sp2 concentration is greatly increased in the resultant carbon nanotube due to graphitization. Annealing eliminated the peak at 13 eV by deprivation of hydrogen. Because the graphitization process lasted only for about 10-3 s in this study, no microstructural details on phase transition from sp3 to sp2 structure during the graphitization were captured in situ by the charge-coupled device (CCD) camera equipped with the TEM, which needs at least about 50 ms (5 × 10-2 s) to take a frame of pictures in TV mode. However, single current data recorded at 50 µs (5 × 10-5 s) revealed details for exploring the variation on the electrical conductance of the hydrocarbon during annealing. Figure 3a is a typical current-voltage curve describing the characteristics during the graphitization. The

J. Phys. Chem. C, Vol. 114, No. 30, 2010 12843 current-voltage curve shows negative conductance and high serration during the annealing process, except for the nonlinear region. Such an irregular variation of current appeared at a current of ∼700 nA (corresponding to a current density of 1.3 × 105 A/cm2) and bias voltage of ∼6.3 V, where the current began to decrease steadily as the bias voltage increased. The current decrease shown in Figure 3a is about 100 nA in magnitude, which is about 14% of the original current 700 nA and only occurred in a very narrow voltage range of about 0.1 V. The current decreased to about 600 nA and became smooth within a voltage range of about 0.2 V. This obvious current reduction and the sustained reduced value did not result from a fluctuation due to noise that occured randomly in the entire measured current which showed many short horn-like protrusions of about 10 nA in magnitude. Usually the noise level in an electrical circuit is only several percent of the working current. The current began to increase at a bias voltage of ∼6.45 V, and then a sharp fluctuation of the current occurred repeatedly when the applied bias voltage increased to ∼6.52 V, resulting in obvious serration. These fluctuations with an average frequency of about 0.025 V are, in magnitude, similar to the current reduction at a bias voltage of ∼6.3 V and possibly resulted from continuous microstructure reconstruction due to transition from the sp3 coordinates to the sp2 coordinates, which could cause variation of electron localization in the amorphous carbon and instantaneous, intricate, and intense redistribution of the space charge layer at the contact and lead to severe electrostatic interaction between the electrode and hydrocarbon including rapid discharge during the in situ measurement. Finally, the current rose very rapidly in a nearly vertical line at a voltage of ∼6.6 V, which indicates the end of graphitization at a current of 1500 nA (corresponding to a current density of about 8.4 × 105 A/cm2 for the resultant nanotube with wall thickness ∼3 nm). After graphitization, the the current-voltage curve became linear. Usually the current density at the end of graphitization is larger than that at initialization, which indicates a transition from semiconductor to conductor. This definite trend was quite consistent across all samples in this study. Figure 3b is a typical current-voltage curve measured for the annealed hydrocarbon. The linear relationship between current and voltage confirms the disappearance of the Schottky barriers at the two contacts and indicates the conductor nature of the graphitized hydrocarbon. After graphitization, the electrical conductance was significantly improved. The current increased drastically with a very small increase in voltage. The electrical resistance of the resultant carbon nanotube was estimated to be about 4 kΩ, corresponding to an electrical resistivity of about 7.8 × 10-4Ω cm which is comparable to the reported electrical resistivity of a graphite sheet although with a 1 order difference in magnitude.38 The relatively large resistance measured in this study may result from contact resistance that cannot be excluded in two-terminal electrical measurements and the interlayer conductance involved in the electron transport process due to the misoriented arrangement of these imperfect graphitic sheets.16 If the graphitization is defined to start when the negative conductance occurred and to end when metallic conduction commenced, the corresponding time interval in current-voltage curve recorded via in situ measurement by TEM technique could be used to accurately examine the duration of the graphitization process. On the basis of the current-voltage characteristics revealed in Figure 3a, the time consumed for the entire graphitization process was estimated to be about 1000 µs (10-3 s) at relatively high accuracy. Meanwhile, important parameters such as the threshold voltage and current for initiating and

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finishing the graphitization of the amorphous hydrocarbon coating can be determined from the measured current-voltage curve. For all measurements, there was a common relationship between the current and applied bias voltage, although there were differences in the critical current and bias voltage values for initializing crystallization between samples because the microstructure, including the size and configuration of the sp2bonded carbon clusters, arrangement of the carbon sp3 and sp2 sites and sp3/sp2 ratio, thickness of the hydrocarbon layer, and contact resistance due to significant differences in the contact area and morphology of individual hydrocarbons and the electrode interface, is sample-specific. 3.2. Effect of Schottky Barrier on the Current-Voltage Characteristics. When a semiconductor was connected to a metal electrode, a Schottky barrier was produced at the interface, which introduced an interfacial resistance. Generally, electrical resistance at the semiconductor and electrode contact is inversely proportional to the contact area,39 provided that other factors such as contact geometry are the same for the contacts. In order to understand the effect of the Schottky barrier on the current-voltage characteristics of the hydrocarbon during graphitization, the hydrocarbon-coated ZnSe nanowires having different contact areas at the two electrodes were annealed in situ, leading to graphitization. As shown in Figure 4a, only one hydrocarbon-coated ZnSe nanowire was touched by the movable probe electrode, and a bundle of hydrocarbon-coated nanowires contacted the fixed electrode, which resulted in different contact areas at the two electrodes. The average thickness of the asproduced hydrocarbon shown in Figure 4a was about 18 nm, nearly the same diameter as that (∼21 nm) of the ZnSe nanowire. Figure 4b shows the morphology of the graphitized hydrocarbon. It is different from the case shown in Figure 1b where the ZnSe nanowires had be completely removed from the resultant carbon nanotubes by thermal evaporation during graphitization. The diameter of a single nanotube touched by the probe electrode decreased to about 47 nm and its average thickness decreased to about 12 nm. Figure 5 is the corresponding current-voltage plot recorded from the in situ-annealed hydrocarbon, where the obviously asymmetrical feature about the zero bias is clearly demonstrated between the bias voltage range from -5 V to 5 V. The negative voltage indicates electrons moving from the hydrocarbon to the fixed ground plate counter-electrode, and the positive voltage indicates electrons moving from the hydrocarbon to the probe electrode. The current increased slowly under the negative bias voltage in comparison to the current under the positive bias. At about 2.7 V, the current rapidly decreased from 12600 nA (corresponding to a current density of about 4.9 × 105 A/cm2) to 3350 nA, only about 27% of the original value. At about 3.9 V, the current increased as shown by a nearly vertical line. Therefore, the graphitization can be thought to begin at ∼2.7 V and end at ∼3.9 V according to the definition in the previous paragraph, which lasted for the bias voltage interval of about 1.2 V. The measured current at the end of graphitization was about 10900 nA (corresponding to a current density of 8.3 × 105 A/cm2). Because the contact area between the single nanowire coated by hydrocarbon and the movable probe electrode was much less than that of a bundle of hydrocarbon-coated nanowires and the fixed electrode, the contact resistance of the hydrocarbon-coated ZnSe nanowire and probe electrode should be larger than that of the other contact. The electrical circuit consisting of a semiconductor squeezed between two electrodes as shown in Figure 4a includes two Schottky barriers arranged in a back to back series at the two interfaces. In this case, the current passing through this circuit

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Figure 4. TEM images depict the morphology of the hydrocarbon coated on the ZnSe nanowire before (a) and after (b) the graphitization. The thickness of the pristine hydrocarbon coating of about 18 nm in panel a was reduced to about 12 nm after graphitization.

was mainly controlled by the Schottky barrier at the reversely biased contact because the Schottky barrier is raised by the reverse bias to prevent current conduction due to further band binding and depletion layer expansion in the semiconductor, which leads to a large threshold voltage.15 Except for the resultant high threshold voltage, the current increased slowly beyond the threshold bias voltage when the reversely biased contact had a large resistance. Similar to that in Figure 3a, the negative conductance in Figure 5 occured at ∼2.7 V and ended at ∼2.8 V and wass observed in a very narrow voltage range of about 0.1 V. There were also some fluctuations with amplitude of about 100 nA just before the end of graphitization. The observed difference in the current under the negative and positive bias conditions in Figure 5 can possibly be explained by the different contact resistances. The high contact resistance at the interface of the hydrocarbon-coated ZnSe nanwire and the probe electrode resulted in a measured current under the


Figure 5. The current-voltage curve measured from the in situannealed hydrocarbon, where an asymmetrical feature resulted from the different Schottky barriers presented at the two contacts between the hydrocarbon and electrodes. Letters S and F indicate the beginning and end of graphitization of the hydrocarbon coating.

negative bias voltage in Figure 5 smaller than the critical value for initialization of graphitization of the hydrocarbon coating in a voltage range of -5 V and, as a result, a current large enough to initialize graphitization of the hydrocarbon can only be obtained when this contact is under forward bias conditions. Upon the basis of the experimental evidence shown in Figures 4 and 5, it is clear that the Schottky barrier at the reversely biased interface can shift the current-voltage curve horizontally via variation of the threshold voltage and alternate the curve shape by a change in gain ratio of the current for per unit bias voltage increment. Consequently, the reversely biased Schottky barrier plays an important role in determination of the critical bias voltage and current for graphitization of the hydrocarbon because it not only affects the threshold voltage for the electrical circuit but also the current passing through the hydrocarbon, leading to a shift in the critical voltage which results in the critical current for initializing the graphitization of the hydrocarbon. When the two Schottky barriers have different heights, the small critical voltage for graphitization of the hydrocarbon can be obtained if the reversely biased Schottky barrier has a relatively low height. On the basis of the current-voltage characteristics revealed in Figure 5, the time used for a whole graphitization process was estimated to be about 3000 µs (3 × 10-3S). In fact, graphitization of the amorphous hydrocarbon is a disorder to order transformation involving some carbons with sp3 tetrahedral coordination being converted to sp2 triangle coordination, nongraphite cluster being converted to graphite cluster, i.e., nucleation of graphite, followed by growth of the graphite nucleus and reconstruction of the graphite sheets. Consequently, such graphitization is a time-consuming process proportional to the sp3/sp2 ratio and mass quantity of hydrocarbon. Therefore, the different times used for the graphitization shown in Figures 3a and 5 can possibly be explained by the different thicknesses of the amorphous hydrocarbon coated on the ZnSe nanwoires, respectively. In Figures 3a and 5 the current-voltage characteristics for the graphitization are quite consistent with each

J. Phys. Chem. C, Vol. 114, No. 30, 2010 12845 other, although the determined threshold voltage and current for initialization and end of graphitization of the amorphous hydrocarbon in Figure 5 are apparently different from those detected in Figure 3a. Because the resultant carbon nanotubes that mainly formed by misoriented graphite sheets are basically similar in microstructure, the different currents for the end of graphitization could possibly be explained by the different wall thicknesses of these carbon nanotubes. However, the different current densities for initialization of graphitization of the hydrocarbon coating can be ascribed to differences in the microstructures of the as-deposited hydrocarbon including size and configuration of sp2 sites, arrangement of the carbon sp3 and sp2 sites, etc. The threshold voltage and current are samplespecific, and slightly scattered values were found in this study because these values seem to be closely related to physical abnormalities such as the interfacial Schottky barrier and the microstructures of the hydrocarbon coating which involve the size and configuration of sp2 sites and arrangement of these carbon sp3 and sp2 sites. It is well-known that the currentinduced Joule heat is proportional to the product of voltage and current flowing through the coating. If a constant Joule heat is needed for initialization of graphitization, hydrocarbon with high resistance will be graphitized at a high threshold value of voltage and a low threshold value of current as a consequence. Because the reported electrical resistivity of hydrogenated amorphous carbon varies from ∼106 to ∼104 Ω cm-1,11,40 scattered values of threshold current and voltage can be expected. 3.3. Effect of Microstructure on the Current-Voltage Characteristics. Because both π and σ bonds are present in amorphous carbon, in terms of the chemical structure, amorphous carbon is distinctly different from many amorphous element semiconductors such as amorphous silicon, in which only σ bonds are present and localization only occurs at the tail states of both conduction and valence bands. The σ bonds of the carbon sp3 sites giving rise to bonding σ and antibonding σ* band states mainly govern the generation of a large gap, which is almost in the same range as that of diamond (∼5 eV). The π bonds of the carbon sp2 sites introduce bonding π and antibonding π* states within the σ-σ* gap41 while pairs, evenmembered rings, and chains of sp2-bonded carbon atoms form π-like states with a smaller gap (∼2 eV).42 In contrast, the isolated sp2 sites and odd-membered rings of sp2-bonded carbon atoms give localized states in the gap. These states form the band edge. The π-bonded clusters may be viewed as wellorganized graphitic units, with an energy gap Eg varying inversely with the number of rings (M) in the cluster described by the following relation:43,44

Eg(eV) )

6 1

M /2

(1)

The band gap gradually decreases as the sp2-bonded carbon cluster coarsens, increasing the electrical conductance. When there are 100 rings in the sp2 cluster (corresponding to a cluster with area of 6.8 nm2 or a cluster with a diameter of about 3 nm), the band gap will decrease to 0.6 eV according to eq 1. Consequently, size and configuration of the sp2-bonded carbon cluster plays an important role in controlling the band gap of the hydrogenated amorphous carbon. As the band gap depends on the size and configuration of the sp2 sites, the growth of the sp2 cluster with increasing annealing temperature is the main reason for the observed band gap narrowing.45 Experimental measurement showed that the band gap of 2.2 eV for the as-

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Figure 6. Schematic diagrams show some special microstructures of the amorphous carbon nets formed by (a) the sp3-bonded carbon mesh including inclusion of the sp2-bonded carbon clusters, (b) sp2-bonded carbon meshes separated by a sp3-bonded carbon band, and (c) sp2-bonded carbon mesh with inclusion of the sp3-bonded carbon clusters, respectively. The dashed lines indicate the sp3-bonded carbons with tetrahedral configuration.

deposited hydrogenated amorphous carbon decreased gradually from 1.9 to 0.8 eV as the annealing temperature increased from 200 to 600 °C.46 When the sp2 clusters coarsened considerably at the late stage of graphitization, the electrical conductance increased significantly as revealed by the in situ current-voltage curve in this study. Because the band structure of the amorphous carbon is mainly determined by the microstructure including size and configuration of the sp2-bonded carbon cluster and arrangement of these carbon sp3 and sp2 sites, the hydrogenated amorphous carbon has a varied band gap depending on the microstructure,47 which can result in the hydrogenated amorphous carbon with obviously different electron transport properties and a strong relationship to the size and configuration of the sp2 site but a relatively weak relationship to the sp3/sp2 ratio. After the hydrogen is removed from the amorphous carbon due to graphitization, a large concentration of sp3 sites facilitates sp2 site migration, resulting in a condensation of sp3 sites. In this case, the amorphous carbon is reconstructed like that of a composite, which comprises the

possible formation of amorphous sp3 mesh and nongraphitic sp2 carbon clusters linked in a random network of sp3 tetrahedral carbons meshed at the initial stage of graphitization;37 thus, the network of the sp3 tetrahedral amorphous carbons is regarded to play an important role as a barrier to electrical conduction. Electron localization present in such an amorphous carbon mesh is important in controlling the electronic properties. If sp2bonded carbon clusters are embedded in the sp3-bonded carbon mesh as shown in Figure 6a, a narrow band gap of the carbon sp2 sites blocked by the wide gap of the carbon sp3 sites can be predicted to result in low conductance for the current passing through the surrounding sp3 layer, although the sp3/sp2 ratio may change considerably for different sizes of sp2 clusters that coarsen during the graphitization. As graphitization proceeded, the microstructure changed both in terms of sp2 content as well as in size and configuration of the sp2-bonded carbon clusters. As shown in Figure 6b, if the coarsened sp2 clusters are still interrupted by the sp3 wall, improvement of the electron condutance can be predicted to be small. The nearly horizontal


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current characteristic in the curve for a voltage range from ∼6.35 V to ∼6.55 V in Figure 2 can possibly be explained by such microstructures. On the contrary, when the sp2 clusters contacted each other to form a mesh with some isolated sp3 clusters as shown in Figure 6c, the transport property highly dominated the band structure of the nongraphite sp2 sites, leading to an obvious increase in current. Therefore, the current-voltage characteristics during graphitization may change considerably depending on the microstructure development, which could result in complex band structures with variable band gap. 3.4. Effect of the Hydrogen Content on the Current-Voltage Characteristics. The electron transport in the hydrogenated amorphous carbon is conventionally attributed to the hopping mechanism where tunneling transitions occur between the locally occupied and unoccupied states at the band tail and the energy difference between the initial and final states is bridged by absorption or emission of a phonon.40 Band tail hopping dominates the electronic transport in amorphous hydrocarbons, and the band tail is assumed to be exponential in amorphous hydrocarbon. As a consequence of the exponential dependence of tunneling rates with distance and jump energy, hopping transport is expected to be very sensitive to the detail of the density of localized states. Therefore, a decrease in current can be explained by an increase in the energy difference between the localized conduction energy (Ec) and valence energy (Ev) or the electron-phonon scattering weakened due to the strain effect in the hydrogenated amorphous carbon caused by a variation of bonding environments resulting from removal of hydrogen from the sp3 sites at the initial stage of graphitization. The disorder in an amorphous material can cause scattering and a reduction in the mobility of the carriers. At finite temperatures, hopping tunneling between localized states is possible. In hopping transport, a broad distribution of localized electronic states, randomly distributed in space, and an isotropic exponential spatial decay exp(-R/L) (where R and L are the spatial distance between an occupied state and an empty state for hopping transition and the localization length, respectively) of the localized state wave functions are usually considered. The transition rate between two sites at energy levels Ei (occupied) and Ej (empty), separated by a distance R, is given in the following equation:48

γij ) ν0 exp(-2R/L) exp[-max(Ej - Ei, 0)/kT]

(2)

where ν0, K, and T are phonon frequency (1013 s-1 in amorphous carbon), Boltzmann constant, and absolute temperature, respectively. Thus, the hopping transition rate is proportional to exp(-2R/L), although there are some other quantities in eq 2 also related to the transition rate. Assuming all other quantities unchanged or varying very slowly at the initial stage of graphitization, it is known from eq 2 that the transition rate γij increases as localization length L increases. A localization length for polymer-like amorphous carbon was estimated to be 1.0-1.5 nm by Silva et al. by monitoring the photoluminescence efficiency as a function of spin density.49 However, the localization lengths reported by other authors are as low as 0.2-0.3 nm for tetrahedral amorphous carbon (TAC) and 0.9 nm for hydrogenated TAC.50 Therefore, the hydrogen can effectively increase state localization. Amorphous carbons are random networks of covalently bonded carbon in hybridized tetrahedral (sp3) and triangle (sp2) local coordination with some of the bonds terminated by hydrogen. The sp3/sp2 ratio of amorphous carbon coating ranges typically from 50% to close to 100%. The hydrogen can be bonded with the carbon so that

a domain of hydrogenated tetrahedral carbon mesh with various bonding structures of sp3-CHn such as sp3-CH3-, sp3-CH2-, and sp3-CH- in hydrocarbons as identified in the FT-IR spectroscopic study of the sp3-CHn feature.51 The hydrogen can also enhance electron transport in the amorphous carbon via passivation of the unpaired dangling bonds at the sp3 and sp2 sites, respectively, and neutralizes the electrically active defects. However, the sp3 sites give a large band gap, and thus the hydrogen bonded to the sp3 sites can only give a minimal contribution to narrowing the band gap as well as transport property since a narrow band gap dictated by the carbon sp2 sites highly dominates the electron transport. Therefore, the high hydrogen content in the amorphous carbon does not mean a high electrical conductance; for example, the electrical resistivity of the hydrogenated amorphous carbon films prepared individually by magnetron sputtering deposition and plasma-enhanced chemical vapor deposition is nearly the same, although these amorphous carbons have different hydrogen contents of 36.5 at.% and 18.1 at.%, respectively.33,34 C-H bonds are relatively easily broken because only one single bond needs to be severed, and hydrogen is a relatively mobile species. Once the bond is broken, the hydrogen may escape from the carbon network, leaving an unpaired dangling bond behind. The unpassivated dangling bonds can depress the electron transport via capture of the electron hopping through these positions. During Joule heating, hydrogen evolved from the microstructure of hydrogenated amorphous carbon. Hydrogen evolution was a continuous process during the heating. Below 300 °C, very little hydrogen evolves, but significant structural reorganization may occur in this temperature region.40 Over 300 °C, hydrogen evolution begins to occur at a much higher rate such that no sp1-CH remains at 460 °C. By 800 °C, over 75% of the total hydrogen within the hydrocarbon has been lost. The sp3-CH is the primary source of hydrogen for effusion, such that molecular hydrogen is formed wherever there are two neighboring hydrogen atoms. Only upon annealing to relatively high temperatures is hydrogen lost from any remaining sp2-CH. The sp3 sites start to convert into sp2 sites under vacuum conditions when the temperature is elevated to about 1400 °C. As a consequence, structural transformations are predicted to occur throughout the heating process until graphite is formed.42,52 Therefore, we have a transition from the as-prepared hydrogenated amorphous hydrocarbon to the olefinic and aromatic ring structures and then to a disordered graphitic ring structure, and finally to microcrystalline graphite. If the hydrogen was driven off the amorphous hydrocarbon at a high rate, the localization length decreased considerably. Provided the statistical distribution of localized electronic states as well as their spatial distance was unchanged when the hydrogen evolved, the hopping transition rate was depressed as the localization length decreased, resulting from hydrogen effusion, which can led to a decrease in the current, consistent with the observed decrease in current at the early stage of graphitization. In fact, the transient charge redistribution due to hydrogen that escaped from both the sp3 and sp2 sites during the graphitization always occurred until no hydrogen was present in the amorphous carbon, but variation may be minimal at the beginning of hydrogen removal because the sp3-CH is the primary source of hydrogen effusion. Therefore, there was no fluctuation of current at the early stage of graphitization. In fact, as the hydrogen evolved from the sp3 sites, the amount of crosslinking between the neighboring carbons and the chain, triangle, and tetrahedral coordinations, respectively, increased due to a decreased number of network terminators, resulting in the

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network constraints. The constraint acts like a hydrostatic stress to make the local carbon atoms become compressed or/and expanded, normally causing the local band gap to widen or/ and diminish. When the hydrogen was removed from the sp2 sites, the band tail states changed significantly due to an increasing number of unpaired dangling bonds in the band gap and at the band edge, which led to variation in the band gap. In this case, the Schottky barrier height could change concomitantly due to variation in the space charge layer near the contact. Because sp3 bonds are converted into sp2 bonds in the amorphous hydrocarbon during graphitization, the band structure varied continuously until the graphite formed. As a consequence, charge redistribution and alternation of Schottky barrier height at the interface between the annealed amorphous hydrocarbon and gold electrodes led to severe static-electric interaction between them. In this case, the transient unstable current including repeated discharge may occur when the bias voltage is elevated, resulting in a highly fluctuating current and an obviously serrated appearance as seen in the current plot. 4. Conclusion Current-voltage characteristics for the graphitization of hydrocarbon deposited on ZnSe nanowires annealed by currentinduced Joule heating have been studied using an in situ TEM technique. On the basis of the measured current-voltage characteristics, the entire graphitization was estimated to be completed within about 10-3 s and is a very rapid annealing process. The critical current and bias voltage values for initializing graphitization varied from sample to sample because the microstructures of the deposited hydrocarbon and contact resistance at the individual amorphous hydrocarbon and the electrode interface are sample-specific. The characteristic trough feature of the current is always present in the measured current-voltage curves, which implies that negative conductance occurred possibly due to an increasing number of the unpassivated dangling bonds and electrically active defects that capture the transport electrons and due to variation in density of the band tail states resulting from the hydrogen that escaped from the sp3 sites in the amorphous carbon network at the early stage of the graphitization. The microstructure development during graphitization of the amorphous hydrocarbon can change the current-voltage characteristics considerably because the band structure is very sensitive to the microstructure variation. The conductance variation revealed in the current-voltage measurements during graphitization of hydrocarbon strongly suggests that the electrical property of the hydrocarbon deposition can be optimized by the proper design of the in situ thermal annealing process for the amorphous carbon. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant Nos. 60796078, 60571044, and 10774174 and partly by “973” National Key Basic Research program of China under Grant No. 2007CB310500, 2007CB 936301 and is acknowledged with gratitude. References and Notes (1) Forrest, R. D.; Burden, A. P.; Silva, S. R. P.; Cheah, L. K.; Shi, X. Appl. Phys. Lett. 1998, 73, 3784. (2) Inoue, T.; Ogletree, D. F.; Sameron, M. Appl. Phys. Lett. 2000, 76, 2961. (3) Hastas, N. A.; Dimitriadis, C. A.; Tassis, D. H.; Logothetidis, S. Appl. Phys. Lett. 2001, 79, 638.

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