Chemical Physics Letters 481 (2009) 224–228

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Chemical Physics Letters

journal homepage: www.elsevier .com/locate /cplet t

Force- and light-controlled electrical transport characteristics of carbonnanotube 1D/2D bulk junctions

Xiao Li, Chunyan Li, Xinming Li, Hongwei Zhu *, Jinquan Wei, Kunlin Wang, Dehai Wu *

Key Laboratory for Advanced Manufacturing by Material Processing Technology and Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China

a r t i c l e i n f o

Article history:Received 15 July 2009In final form 28 September 2009Available online 1 October 2009

0009-2614/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cplett.2009.09.097

* Corresponding authors. Fax: +86 10 62770190.E-mail addresses: hongweizhu@tsinghua.edu.cn (H

edu.cn (D. Wu).

a b s t r a c t

Pure carbon bulk junctions are fabricated based on carbon nanotube (CNT) macrostructures and theirelectrical transport characteristics are investigated. The planar 1D/2D strand-on-film (SOF) junctionsshow interesting force- and light-controlled transport behaviors. Considering the excellent chemical sta-bility and good mechanical properties (strength, hardness and elasticity) of CNTs, the SOF junctions couldfind practical and wide applications in electromechanical and optoelectronic devices.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Bulk junctions made of low dimensional materials are of greatimportance in the development of high efficiency solar cells andrelated optoelectronic devices. To establish a rational connectionbetween the microscopic characteristics of nanomaterials withtheir macroscopic performance, research is now underway forsearching new photoactive materials and novel fabrication tech-niques. One emphasis of current concerns is to eliminate the inter-vention of conventional semiconductors (e.g. silicon). To this end,it is imperative to find a new kind of materials to provide efficientphoton excitation or light energy conversion. In the search foralternative materials, carbon is highly attractive because it is ex-pected to have more diversified properties comparing with siliconand would be highly stable [1,2]. For example, diamond like amor-phous carbon (a-C) has been used partially as alternative materialof silicon in photovoltaic devices thanks to the feasibility of con-trolling their conduction type. a-C/Si [3–7] and a-C/GaAs [8] het-erojunction photovoltaic cells have been realized throughstandard deposition and doping techniques and show promisingpower conversion efficiencies of up to 8%. Carbon nanotube(CNT) based devices showed many interesting characteristics use-ful for solar cells production such as high mobility, fairly highminority carrier diffusion length, high absorption coefficient anddirect energy gaps. CNT thin films have been directly configuredas energy conversion materials to replace p-type silicon to createheterojunctions with n-type silicon [9–12]. Initial tests have showna conversion efficiency of 5–7% at AM1.5. CNTs can also form a rec-tifying heterojunction with n-GaAs [13] with an efficiency of up to

ll rights reserved.

. Zhu), wdh-dme@tsinghua.

3.8% under the illumination of a green laser or desk lamp. Theseresults show the possibility to produce CNT-based photoelectricdevices with appreciable performance while reducing the use of,or eliminating entirely, the traditional semiconductor layer.Recently, considerable attention has also turned to lightweight,flexible thin film photoelectric devices with reducing cost orincreasing efficiency. The first step toward this direction is to real-ize bulk junctions made of sole carbon as the basic building blockof all-carbon devices.

Among the intense studies on CNT-based Schottky junctions,p–n junctions or heterojunctions, crossed CNT (CC) junctions areof particular importance due to their special interface (Fig. 1A) ofsmall contact area and force-controlled intertube conductance[14,15]. Previous studies have addressed certain aspects of thestructure, electronic and photoelectric properties of model and realsystems involving CC contacts. Potential applications of CC junc-tions include non-volatile memory [16], electromechanical devices[17,18] and field effect transistors [19–21]. A more recent workconsidered a geometry in which the relative stability, structure,and conductance of CC junctions are explored [22]. Spatially re-solved potential distribution in CC junction devices has also beensystematically studied using scanning photocurrent microscopy[23]. In these studies, however, only individual CNTs have beeninvestigated. As mentioned above, to develop and get a compre-hensive understanding of all carbon macroscale devices, a morecomprehensive study on bulk CNT-based junctions is of particularnecessity.

Here we give the first demonstration of the pure carbon bulkjunctions based on CNT macrostructures and present the fabrica-tion and characterizations of Schottky devices operated at roomtemperature. One-dimensional (1D) strands and two-dimensional(2D) films of single-walled CNTs (SWNTs) are chosen as both themetallic and semiconducting components (Fig. 1B). The planar

Fig. 1. Structural characterizations of the CNT SOF junction. (A) Models for crossed CNT junction and (B) bulk SOF junction. (C) Schematic diagram and (D) optical images ofthe SOF junction. (E) Low and (F) high resolution SEM images of the CNT thin film. (G, H) Low and (I) high resolution SEM images of the CNT strand.

X. Li et al. / Chemical Physics Letters 481 (2009) 224–228 225

1D/2D strand-on-film (SOF) junction shows interesting force- andlight-controlled transport responses. Considering the excellentchemical stability and good mechanical properties (strength, hard-ness and elasticity) of CNTs, the SOF junctions could find practicaland wide applications in electromechanical and optoelectronic de-vices. This work aims to tackle the issues mentioned above fromboth basic research and engineering application perspectives. Junc-tions made of sole carbon will help us to explore more complexsystems in the future.

2. Experimental

SWNT films were prepared by a floating chemical vapor deposi-tion technique with a liquid-free precursor: a solid volatile mixtureof ferrocene and sulfur (atomic ratio Fe:S = 10:1) [24]. As preparedSWNT film was subjected to a post purification which employed acombination of oxidations (heat in air at 450 oC for 1 h or immersein 30% H2O2 solution for 72 h) and rinsing with hydrocholoric acid(37% HCl). Smooth and homogenous SWNT thin films were ob-tained by a post treatment combining the purification, sidewall

functionalization mentioned above with water/ethanol competi-tion. When ethanol is dropped into the water, ethanol moleculesspread quickly onto the water surface forming a Langmuir mono-layer. The formation of SWNT thin film (20–100 nm thick) occursfollowing the dispersion of the ethanol layer along the water sur-face and could be easily collected and formed a conformal coatingon a target substrate. SWNT strands were obtained by a simplefilm-to-fiber conversion technique. A strand was drawn out fromthe SWNT film under stretching to improve the alignment of thenanotube bundles. The SOF junction was then formed as illustratedin Fig. 1C. Copper foils were used as electrodes and silver paste wasintroduced to ensure the Ohm contact between CNTs and the elec-trodes. SWNT strands and films used for SOF junctions are approx-imately 1 cm long and wide.

The samples were characterized by scanning electron micros-copy (SEM, Hitachi S3600 N). Electrical transport was character-ized by using a Keithley 2602 SourceMeter. The current wasmeasured from zero bias to 4 V, then applied voltage was succes-sively decreased to zero. Similar procedure was followed in the re-verse bias region. The measurements were repeated till stablevalues were reached.

226 X. Li et al. / Chemical Physics Letters 481 (2009) 224–228

3. Results and discussion

Fig. 1D shows a top view of a typical SOF junction fabricated ona glass slide based on the schematic diagram illustrated in Fig. 1C.The underlying CNT thin film presents the web-like networking ofinterlinked bundles composed of self-assembled SWNTs (Fig. 1Eand F). The strands are generally 20�100 lm thick, consisting ofaxially aligned SWNT bundles (Fig. 1G and H). SWNT strands andfilms have similar chirality distributions as they are obtained fromthe same batch of purified sample.

As shown in Fig. 2A, a typical I–V curve for a SOF junction re-veals an asymmetric transport behavior. It is known that bulkCNT samples consist of a mixture of metallic and semiconductingnanotubes with random chirality distribution. Therefore, fromthe aspect of statistics, the metallic and semiconducting tubes(slightly p-doped) in both strand and films can interact with each

Fig. 2. Electrical transport characteristics of the SOF junction. (A) I–V curve. Inset: biasresistance (R). (C) SM and MS junctions. (D) An enlarge view of (B) and (C). (E) Representavalence bands, respectively.

other and form four types of intertube junctions (both crossedand parallel) at the SOF interface: SM, MS, SS and MM, where S de-notes the semiconducting phase, M denotes the metallic phase andthe first Letter represents the phase in the strand, i.e., SM repre-sents the junction between the semiconducting tubes in the strandand the metallic tubes in the film. For individual CNTs, the semi-conducting nanotube is depleted at the SM or MS junction by themetallic nanotube, forming a rectifying Schottky barrier [14,15];while the MM and SS junctions are good tunnel contacts, showinglinear transport. Hence, the SOF bulk junction shown in Fig. 2A canbe further divided into three parts: an Ohm contact at the junction(MM + SS), the intra-strand and intra-film resistance (R) and ananti-parallel junction pair containing two Schottky junctions withopposite polarities (SM + MS).

The non-linear I–V characteristic of the SOF junction can be ex-pressed by the thermoionic emission model

is applied to the strand. (B) MM, SS junctions plus the intra-strand and intra-filmtive signs and band diagrams. EF, EC, EV stand for the Fermi level, the conduction and

Fig. 4. Light-controlled electrical transport characteristics of the SOF junction.

X. Li et al. / Chemical Physics Letters 481 (2009) 224–228 227

I ¼ Is exp�eVnkT

� �� 1

� �¼ AA�T2 exp

�eUb

kT

� �exp

�eVnkT

� �� 1

� �;

ð1Þ

where A is the contact area, A* is the effective Richardson constant, Tis absolute temperature, e is electronic charge, Ub is the barrierheight, k is the Boltzman constant, n is the ideality factor. Is is thereversed saturated current which is negligible. The slope of the lin-ear part of the curve gives the values of ideality factors.

The curve fitting was carried out by first subtracting the linearpart obtained near zero bias then fitting with Eq. (1). The corre-sponding linear and dual rectified transporting I–V curves areshown in Fig. 2B and C. At low bias, the conductance of MM/SSjunctions is far beyond that for SM/MS junctions (see Fig. 2D, anenlarged view of Fig. 2B/C). At high bias (>2 V), a relatively largercurrent is measured for SM/MS junctions. This might arise fromthe increasing reverse leakage current or/and the early occurrenceof the breakdown of the Schottky junction which is also confirmedby the presence of two breakdown points at high bias shown inFig. 2A. The SOF Schottky junctions have ideality factors of 25–35, which are still far from that for the ideal diodes due to the weakp-doping of semiconducting SWNTs. Fig. 2E shows the representa-tive signs for these junctions and corresponding energy band dia-grams by which the phenomena shown in Fig. 2B and C can bequalitatively explained. In the SM or MS junction, the non-linearityof the I–V curve can be ascribed to the Schottky barrier (Ub) locatedon the semiconducting side. In the MM and SS junctions, the linearcharge transport results from a finite density of states at the Fermisurface on both sides of the junction.

It is further confirmed that the contact force between the strandand the film has a significant influence on the electrical transportbehavior of the SOF junction from the force-controlled conduc-tance measurements. We applied a compressive force up to50 mN to the strand, and recorded the transport characteristicsof the SOF junction. A non-linear-to-linear transition is clearlyidentified from the I–V curve development as the applied compres-sive force is increasing (Fig. 3A). When the applied force is less than5 mN, the I–V curves show a typical non-linear dual Schottky rec-tification feature (Fig. 3B and region I of C), similar to that shown inFig. 2. This has been explained by the presence of a finite intertubepotential barrier that impedes ballistic electron transport betweentubes. As theoretically predicted, for individual CNTs, the most sta-ble junction geometry has the smallest conductance which in-creases rapidly as force is applied [22]. However, this condition ishardly to be met in our case due to the random distribution ofnanotube chirality and homogeneous arrangement of the tubebundles within the underlying film. As the applied force is further

Fig. 3. Effect of the applied compressive force on the electrical transport of the SOF junctiinset shows the schematic for strand-film interaction under strong applied force. (B) Anfilm interactions under weak (upper-left) and relatively strong (lower-right) applied for

increased, the CNTs deform significantly at the junction. Thecharge density in the contact region becomes sizeable as the CNTsbecome closer and more deformed, resulting in a significant over-lap of intertube wave function. This yields an increasing conduc-tance (region II of Fig. 3C) and a non-linear-to-linear transition.As the applied force is up to 20 mN, the conductance reaches itssaturated state (region III of Fig. 3C) and the corresponding I–Vcurves show a completely linear Ohm contact behavior. It has beentheoretically confirmed that for crossed intertube junctions the C–C bonds interlinking the tubes can form with increased contactforces [22]. Therefore, the potential barrier disappears under high-er applied forces. When the external force is released, the I–V curveis recovered accordingly with minor hysteresis, suggesting that theradial deformations of tubes and bundles have been recoveredafter the external force has been removed. This result differs fromthe theoretical predication that the interlinking bonds survive evenafter the contact forces are released and whole structure is fully re-laxed [17,18]. The force-controlled reversible transporting charac-teristics indicate the possibility and importance of the SOFjunctions in electromechanical device applications.

The photo-response of the SOF junction is further investigated.Instead of mapping the photocurrent distribution, the whole junc-tion is evaluated under the illumination of spectrum and powerdensity comparable with AM1.5. As shown in Fig. 4, the SOFjunction operated at 10 mN shows a distinct photodependentnon-linear effect. The transport characteristics (both conductanceand I–V curve shape) are reversible and can be well tuned byvarying the intensities of the incident light. For example, underthe illumination of the 50 mW/cm2 light, the I–V curve unambigu-ously shows the light modulated non-linearity of the SOF junction.

on. (A) I–V curves with increasing applied force (indicated as the dashed arrow). Theenlarge view highlights the non-linear I–V curves. Insets: the schematics for strand-ce are shown. (C) Conductance profile as a function of applied force.

228 X. Li et al. / Chemical Physics Letters 481 (2009) 224–228

The decreased conductance at higher incident intensities can bepartially attributed to the heating effect which increases theintra-strand and intra-film resistance. While the physical mecha-nism of the variation and photodependence of the transport charac-teristics of the SOF junction upon illumination remains to beinvestigated, this light-controlled performance of the anti-parallelSchottky junctions does suggest the application of the SOF junctionas a light sensor or a light-controlled rectifier. With our new exper-imental observation, the devices can be constructed much moreeasily in a pure carbon manner in macroscale. The level ofperformance could be adjusted by choosing the CNT strands withappropriate dimensions or by varying the contact length betweenstrand and film.

4. Conclusions

To conclude, we report for the first time the bulk junctionsbased on CNTs. It has been revealed that the SOF interface give riseto an anti-parallel pair of Schottky junctions. The force- and light-controlled behaviors provide deep insights of future application ofpure carbon devices in electromechanical and light-controlled de-vices. Future focus is to develop optimal design of CNT-based het-erojunctions, including band gaps modulation, control of bandstructure, rectification behavior and interface buffer layer. Theemphasis will be on the SOF junction performance on flexible sub-strates and its possible use as photovoltaic cells upon functionali-zation and cooperation with other photoactive materials. It isexpected that after complete development, pure carbon devicesmay become more economical than silicon-based devices, andthe cost would be extremely low and unlike silicon, it is highlychemically and environmentally stable.

Acknowledgements

This work is supported by National Science Foundation of China(#50972067), Program for New Century Excellent Talents inUniversity (#NCET-08-0322) and the Scientific Research Founda-

tion for the Returned Overseas Chinese Scholars (#20091020304),State Education Ministry of China.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cplett.2009.09.097.

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