Physica E 74 (2015) 451–456

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Physica E

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Fabrication of thin-film transistor based on self-assembledsingle-walled carbon nanotube network

Xiaodong Liu, Changxin Chen n, Poonam Sharma, Liying Zhang, Ying Wang, Nantao Hu,Chuanjuan Song, Liangming Wei, Qinran Liu, Yafei ZhangKey Laboratory for Thin Film and Microfabrication of the Ministry of Education, National Key Laboratory of Science and Technology on Micro/Nano Fab-rication, Department of Micro/Nano Electronics, School of Electronic, Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai200240, PR China

H I G H L I G H T S

G R A P H I C A L A

� Developing a method to preparecarbon-nanotube network usingself-assembly technique.

� Investigating the effect of as-sembly parameters on the den-sity of nanotube network.

� Fabricating thin-film transistorsbased on assembled monolayerSWNT network.

� Studying the dependence be-tween nanotube density andtransistor characteristics.

x.doi.org/10.1016/j.physe.2015.07.03677/& 2015 Elsevier B.V. All rights reserved.

esponding author.ail address: chen.c.x@sjtu.edu.cn (C. Chen).

B S T R A C T

a r t i c l e i n f o

Article history:Received 9 June 2015Received in revised form15 July 2015Accepted 29 July 2015Available online 30 July 2015

Keywords:Thin film transistorsSWCNTsSelf-assembly

a b s t r a c t

Thin-film transistor based on controllable electrostatic self-assembled monolayer single-wall carbonnanotubes (SWNTs) network has been fabricated by varying the density of nanotubes on the siliconsubstrate. The densities of SWNTs network have been investigated as a function of concentration andassembly time. It has been observed that the density of SWNTs network increases from 0.6 mm�2 to2.1 mm�2, as the average on-state current (Ion) increases from 0.5 mA to 1.47 mA. The device has a currenton/off ratio (Ion/Ioff) of 1.3�104 when Ion reaches to 1.34 mA.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Attributing to the unique 1D and defect-free structure, single-wall carbon nanotubes (SWNTs) possess high carrier mobility andlow transport scattering [1–4]. SWNTs are used as channels infield-effect transistors (FET) [5–7]. However, the maximum cur-rent-carrying capacity of SWNTs is �25 mA per tube [5], whichconfines to the on-state current (Ion) and trans-conductance (gm)

of carbon nanotube field-effect transistor (CNT-FET). CNT-FET haspotential in the other fields, such as switch for detecting the highcurrent through power transistors, flexible electronics devices andsensors etc. [8]. Also, SWNTs network is commonly used as aconductive channel to ensure that the device has large outputcurrent, trans-conductance and small on-state resistance. On thecontrary, the fabrication of large-scale devices with specific controlover the positioning of CNT building blocks is quite challenging,due to intrinsic difficulties faced in manipulating individual na-notubes [9]. Besides, the technical limitations faced during fabri-cation and purification of SWNTs, the metallic characteristic of

X. Liu et al. / Physica E 74 (2015) 451–456452

SWNTs has not eliminated completely. This metallic characteristiccauses the failure of individual or aligned SWNTs in the devices,which furthermore decreases the performance. In order to fix thisproblem faced during assembly of device, the 2 Dimensional (2D)networks of SWNTs are randomly distributed. This method is ad-vantageous over the statistical averaging for better reproducibility.Since decades, deposition methods such as vacuum filtration,chemical vapor deposition (CVD), spin coating, dip coating, spraycoating, and electrophoretic have been widely used by the re-searchers [10–18] for the deposition of SWNTs network betweenthe source and drain electrodes of device. However, high tem-perature operation and irrepressible monolayer deposition be-tween electrodes are the biggest drawbacks accompanying thesemethodologies.

The present communication describes an economical and facileway to solve the above problems in the fabrication of the FETsbased on SWNT networks. Electrostatic self-assembly technique isused for the fabrication of uniform SWNTs network thin film onsilicon substrate. The present fabrication technique is advanta-geous over other techniques due to controlled growth rate ofmonolayer SWNTs network and higher final yield. Palladium (Pd)electrodes, fabricated on SWNTs network by lift-off process, re-sulted in excellent electrical contacts [19]. The densities of SWNTsnetwork are well controlled by varying the concentrations ofSWNTs in aqueous suspensions and hold-up assembly time. Theelectrical properties of CNT-FET with variable SWNTs networkdensity have been examined. This methodology is considered to beeffective in solving many problems confront in the field of CNTbased transistors.

2. Experimental methodologies

2.1 Acidification of SWNTs

SWNTs (IsoNanotubes-S procured from NanoIntegris) are sus-pended in the piranha solution, a mixture of 30 ml of 98% H2SO4

and 10 ml of 60% HNO3. The suspension is heated at 50 °C undervigorous magnetic stirring. On continuous stirring for next 12 h,the resulting solution is diluted with deionized (DI) water andfiltered using 95 sand core funnel. After that, the pH value of thesuspension is measured. If the pH value of the suspension is loweror higher than 6, the dilution step is repeated again until the pH

Fig. 1. Schematic of self-assembling mono-layer SWNT network, including the followinpiranha solution to obtain the -OH group on the wafer surface (b); Step 2, the –NH2 grouisopropanol solution (c); Step 3, the resulted wafer was immersed in the SWNTs solutio

value of solution reaches to 6. On achieving required pH value, theresultant filtrate is vacuum dried at 60 °C for 24 h.

2.2. Hydrophilic treatment of silicon wafer surface

The surface of silicon wafer is hydrophobic in nature; thus, thehydrophilic treatment is required. Before starting any treatmenton the silicon wafer, the wafer is ultrasonically cleaned step-wiseusing acetone, ethyl alcohol and DI water, to remove all the im-purities presented on its surface. After that, Si wafer is immersedin piranha solution (98% H2SO4:30% H2O2¼3:1 (v/v)) and kept at80 °C for 80 min. This treatment favors the attachment of hydroxylgroups (–OH) onto the silicon wafer surface, thus promoting thefunctionalization of wafer (Fig. 1, step 1(a)–(b)). At last, the waferis washed thoroughly with DI water to remove impurities anddried under nitrogen atmosphere.

2.3. Functionalization of silicon wafer surface via amine group

Functionalization of amine group on the silicon wafer surface isachieved by hydrolytic reaction between hydroxyl group, presenton the surface of hydrophilic silicon wafer, and aminopropyl-trimethoxysilane (APTES), as illustrated in Fig. 1(c). The hydro-philic silicon wafer is suspended in the APTES solution((CH3)2CHOH:APS¼20:1(v/v)) for 20 min. After that, the siliconwafer is washed with DI water and dried under nitrogen atmo-sphere. Finally, the amine group functionalized silicon wafer issubjected for annealing at 120 °C for 10 min.

2.4. Electrostatic self-assembly of SWNTs network

The self-assembly of SWNTs network onto silicon surface ba-sically depends on the electrostatic interaction between carboxylgroup, carrying negative charges on SWNT side walls, and aminogroup, carrying positive charges, present on the surface of SiO2

substrate. The aqueous suspensions of SWNTs have been preparedwith three different concentrations, such as 0.1, 0.5 and 1 mg/ml.After that, amine group treated silicon wafers are immersed in allthree SWNTs suspensions for 20 min (Fig. 1(d)). Later on, treatedwafers are washed with DI water and dried in nitrogen atmo-sphere. The effect of SWNTs deposition time on the density ofSWNTs network onto wafer surfaces is examined via varying thewafer suspension time in 2 mg/ml SWNTs solution from 10 min to120 min.

g steps: Step 1, the Si wafer with 50-nm SiO2 (a) was heated at 80 °C for 80 min inps were attached to the wafer surface by immersing the treated wafer using APTES/n to conduct the electrostatic self-assembly of SWNTs networks.

X. Liu et al. / Physica E 74 (2015) 451–456 453

2.5. Fabrication of CNT-FETs

A 50 nm thick layer of SiO2 is deposited on the silicon wafersurface refereeing to the schematic of device in other section. Awell-distributed self-assembled network of SWNTs is deposited onthe surface of silicon substrate. Techniques, such as lithographyand magnetron sputtering are used to fabricate the pair of parallelPd electrode, of approximate thickness 100 nm, onto the as-sembled SiO2 substrate. The deposited electrodes are 6 mm longand 20 μm wide apart.

3. Results and discussion

3.1. Characterization of acidic-SWNTs

As it is well known, the SWNTs are insoluble in aqueous solu-tion. After carbonyl group treatment, the solubility of SWNTs in-creases upto certain extent which is clearly observed from insetimage shown in Fig. 2(a). Fig. 2(a) represents the Fourier Trans-form Infrared (FT-IR) spectrum (FT-IR model Vertex 70) of acid-ified-SWNTs. Sharp peaks centered at 1722 cm�1 and 1595 cm�1

attributed to carboxylic (–COOH) group and vibration mode ofcarbonyl (C¼O) group, respectively, have been clearly observed toconfirm the efficacious intercalation of carboxyl group with theSWNTs surface after acid treatment. Peak centered at 3400 cm�1 isalso observed in the scan, which is attributed to the presence of –OH group on the wafer surface.

In order to further clarify the successful acidification of SWNTssurface, quantitative analysis is carried out by acid–alkali titrationmethod [20]. The process steps for acid–alkali titration is asfollows:

1. 20 mg of acidified-SWNTs are dispersed in 25 ml of 0.05 mol/LNaHCO3 solution and magnetically stirred for 48 h.

2. 10 ml of 0.05 mol/L HCl standard solution is added slowly inabove suspension.

3. The suspension, after that, is titrated using standard NaOH so-lution (1.6 ml, 0.05 mol/L) using Phenolphthalein as anindicator.

The number of carboxyl groups present on the surface of silicon

Fig. 2. (a) FT-IR spectra of acidic SWNTs (inset shows the SWNTs aqueous solution)interpretation of the references to color in this figure legend, the reader is referred to t

wafer is calculated using the following equation:

CM

M100%

1COOH

C= ( ) ×

( )−

where C is the total number of carboxyl group, M–COOH is thequantity of carboxyl group present on the surface of acidic-SWNTs,and MC is the number of carbon atoms in acid-SWNTs.

The comparative studies on pristine SWNTs and acidic SWNTsby acid–alkali titration analysis have, also, been investigated in thepresent communication keeping 0.1 ml of NaOH solution as stan-dard for pristine SWNTs. After the titration analysis, the traces ofcarboxyl group, i.e. 0.3%, are still present on the surface of pristineSWNTs that are attributed to the presence of defects on the SWNTssurface. These defects lead to the high reactivity of SWNTs surfacethat reacts with free oxygen present in the ambiance to produceacidic group. Moreover, the number of carboxyl group on SWNTssurface increases to 4.8% after acidic treatment.

To get the additional insight for determining the number of –COOH group present on the surface of SWNTs, Thermogravimetric(model PerkinsElmer, Pyris 1 TGA) analysis is done. Fig. 2(b) showsthe TGA curves of SWNTs before and after acid treatment in Aratmosphere. The curves clearly indicate that the pristine SWNTslose 3% of its weight at 600 °C, attributed to the presence ofamorphous carbon on it. However, acidified-SWNTs lose 17% of itsweight at 600 °C. Considering the relative molecular mass of –

COOH as 45, this is equivalent to its mole fraction value 4.5%. Theseresults are well consistent with results obtained from acid–alkalititration.

3.2. Contact angle measurement of silicon wafer surface

Our research indicates that the contact angle value of the siliconwafer surface decreases from 95° to 40° after hydrophilic treatment(graphs provided in supplementary material). This behavior is at-tributed to the existence of hydroxyl (�OH) groups on the surfaceof wafer, generated (Fig. 1(b)) after hydrophilic treatment. The hy-drolytic reaction between the hydroxyl group, present on siliconwafer surface, and APTES makes covalent bonding (Fig. 1(c)). Posi-tive charges are present on the silicon wafer surface due to theexistence of amine group (–NH2) as Lewis base. After the aminegroup treatment, the contact angle value increases to 62° resultingin efficacious add-on of amine group on the wafer surface.

and (b) TGA curves of SWNTs before (black) and after (red) acid treatment. (Forhe web version of this article.)

X. Liu et al. / Physica E 74 (2015) 451–456454

3.3. Monolayer SWNTs network self-assembly

The growth rate of SWNTs network, deposited on the surface ofwafers, is measured as a function of wafer suspension time. Threedifferent concentrations, such as 0.1, 0.5 and 1 mg/ml, of SWNTsaqueous solution have been prepared and wafers are suspended inthose solutions for 20 min. The deposited SWNTs densities areexamined employing scanning electron microscopy (Model ZeissUltra 55) shown in Fig. 3(a)–(c). It is clearly observed from theelectron micrographs that the density of SWNTs networks in-creases effectively on increasing the concentration of SWNTs in theaqueous suspension medium. The influence of assembly time onSWNTs network density has also been investigated for 0.5 mg/mlSWNTs suspension concentration, shown in Fig. 3(d)–(f). Withincrease in suspension time from 10 min to 30 min and later to60 min, the SWNTs network grows onto the wafer surface andbecomes denser. On further increasing the suspension time from60 min to 120 min, no further increase in the growth rate ofSWNTs network density is observed, confirming the arrival ofgrowth saturation point.

3.4. The electrical characterizations of SWNT-TFT

Fig. 4(a) and (b) shows the schematic of device and scanningelectron micrographs (SEM) of the SWNTs network along with Pdelectrode pair. From the images, it is clear that the mono-layer ofSWNTs is evenly distributed on the wafer surface. SWNTs havingan average length of �3 mm overlapped each other forming se-parate paths. Fig. 4(c) shows the transfer characteristic of a typicaldevice fabricated with lowest density of SWNTs network atVDS¼�1.4 V to �0.2 V. The step size for every measurement is0.4 V. The value of Ion of the device, at VG¼ �8 V, obtained is0.17 mA. The maximum trans-conductance (gm), defined as ΔIDS/ΔVDS in the tested VG range at VDS¼1.4 V, is 150 μS. A high currenton/off ratio (Ion/Ioff) of 6.5�104 is obtained. Fig. 4(d) shows thetransfer characteristic of a typical device fabricated with highdensity of SWNTs network at VDS¼�1.4 V to �0.2 V, having stepsize of 0.4 V. On increasing the density of SWNTs network, Ion ofthe device, at VG¼�8 V when VDS¼�1.4 V, is observed to be1.34 mA. The trans-conductance (gm) in the tested VG range, at

Fig. 3. SEM images of SWNTs network on the Si wafer, 20 min suspension time: (a) 0.1 mSi wafer substrate (0.5 mg/ml) (d) 10 min, (e) 30 min, and (f) 60 min.

VDS¼�1.4 V, is 750 μS. A high current on/off ratio (Ion/Ioff) of4.2�103 is obtained in the present investigation.

The transfer characteristics of 24 devices at different SWNTsnetwork density are examined. Fig. 5 shows the effect of SWNTsnetwork density on Ion and Ion/Ioff. As the density of SWNTs net-work increases from 0.6 mm�2 to 2.1 mm�2 the average Ion in-creases from 0.5 mA to 1.47 mA. The Ion attained percolationthreshold when the ρs value reaches above 1.4 mm�2. This value isconsistent with the theoretical value as given in the literature

Lc4.2362

cρ =

π, where Lc is the average length of SWNTs [21]. ρs–Ion

curves are Gaussian fitted, in Fig. 5, in order to study the effect ofthe ρs on Ion before attaining saturation point. The relation be-tween ρs and Ion is given by

I intercept B B 2on s 2 s2ρ ρ= + + ( )

where the values of intercept, B, and B2 are 0.91, �1.56, and1.46, respectively. Drastic increase in Ion value is observed beforethe arrival of percolation threshold. This behavior might be due tothe increase in the conduction channel on the wafer surface as thedensity of SWNTs network increases. When the value of ρs in-creases higher than 1.4 mm�2, the Ion attained percolationthreshold. This is due to the reason that all the SWNTs are inter-connected and conduction channel reaches to saturation point. Onthe other hand, Ion/Ioff value exhibits the reverse trend of decreasein the value with an increase in SWNTs network density. The fit-ting function follows:

II

Ae3

on

off

st s=

( )

ρρ

−( )

where the value of A is 105, t(ρs) is the constant and function ofρs. The present graphical trend in Fig. 5 is observed due to theexistence of additional electronic paths with increase in the na-notube densities [22]. From Fig. 5, it has been observed that thedecrease in Ion/Ioff does not exceed one order of magnitude withthe increase in the Ion. The optimal values of the Ion and Ion/Ioff

g/ml, (b) 0.5 mg/ml, and (c) 1 mg/ml and the effect of the self-assembly time on the

Fig. 4. (a) Schematic of device (W¼20 mm, Lch¼6 mm) and (b) SEM image of grown SWNTs network between source and drain electrodes, (c) and (d) shows the transfercharacteristics of a typical SWNT-TFT fabricated with low and high SWNTs network density, respectively. Inset of (c) and (d) shows the logarithmic plot of transfer char-acteristics at VDS¼�1.4 V.

Fig. 5. Represents Ion (black) and Ion/Ioff (blue) for the SWNT-FET with differentSWNTs network density, (ρs measures in μm�2). Ion/Ioff is obtained from the transfercharacteristics for the device at a drain voltage (VDs) of �1.4 V. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

X. Liu et al. / Physica E 74 (2015) 451–456 455

obtained are 1.34 mA and 1.3�104, respectively, when ρs is1.4 mm�2.

4. Conclusions

High performance monolayer SWNTs network transistor isfabricated by controlled self-assembly technique. Acidified SWNTs

assembled onto the amine group treated SiO2 surface and mono-layer of SWNTs network are uniformly distributed. The distribu-tion density of SWNTs network is controlled on varying the con-centration of SWNTs suspension and assembly time. The functiondependency of the Ion and Ion/Ioff on the ρs is presented for thisassembled SWNTs network FETs. The on-state current reaches to1.34 mA, while Ion/Ioff retains the value of 1.3x104. The presentinvestigation provides valuable direction in the field of large-scaleproduction of SWNTs transistors.

Acknowledgments

This work is supported by the National Natural Science Foun-dation of China (Nos. 61177052 and 60807008); A Foundation forthe Author of National Excellent Doctoral Dissertation of China(FANEDD) (No. 201154); Program for New Century Excellent Ta-lents in University (No. NCET-11-0319); Shanghai Pujiang-TalentProgram (No. 15PJ1403300); Fok Ying-Tong Education Foundationfor Young Teachers in the Higher Education Institutions of China(No. 131064); and the National Natural Science Foundation ofChina (No. 51272155).

References

[1] T. Durkop, et al., Extraordinary mobility in semiconducting carbon nanotubes,Nano Lett. 4 (1) (2004) 35–39.

[2] J.C. Charlier, X. Blase, S. Roche, Electronic and transport properties of nano-tubes, Rev. Mod. Phys. 79 (2) (2007) 677–732.

[3] D.A. Li, et al., Intrinsic carrier mobility of a single-layer graphene covalently

X. Liu et al. / Physica E 74 (2015) 451–456456

bonded with single-walled carbon nanotubes, J. Appl. Phys. 115 (23) (2014)233701.

[4] Y.S. Li, et al., Contactless probing of the intrinsic carrier transport in single-walled carbon nanotubes, Nano Res. 7 (11) (2014) 1623–1630.

[5] S.J. Tans, A.R.M. Verschueren, C. Dekker, Room-temperature transistor basedon a single carbon nanotube, Nature 393 (6680) (1998) 49–52.

[6] Q. Cao, et al., Origins and characteristics of the threshold voltage variability ofquasiballistic single-walled carbon nanotube field-effect transistors, ACS Nano9 (2) (2015) 1936–1944.

[7] Q. Cao, et al., Arrays of single-walled carbon nanotubes with full surfacecoverage for high-performance electronics, Nat. Nanotechnol. 8 (3) (2013)180–186.

[8] K.S. Karimov, M.T.S. Chani, F.A. Khalid, Carbon nanotubes film based tem-perature sensors, Phys. E: Low-Dimens. Syst. Nanostruct. 43 (9) (2011)1701–1703.

[9] A. Saha, C. Jiang, A.A. Marti, Carbon nanotube networks on different platforms,Carbon 79 (2014) 1–18.

[10] B. King, B. Panchapakesan, Vacuum filtration based formation of liquid crystalfilms of semiconducting carbon nanotubes and high performance transistordevices, Nanotechnology 25 (17) (2014) 175201.

[11] Z.J. Han, et al., SWCNT networks on nanoporous silica catalyst support: mor-phological and connectivity control for nanoelectronic, gas-sensing, and bio-sensing devices, ACS Nano 6 (7) (2012) 5809–5819.

[12] J.W. Jo, et al., Fabrication of highly conductive and transparent thin films fromsingle-walled carbon nanotubes using a new non-ionic surfactant via spin

coating, ACS Nano 4 (9) (2010) 5382–5388.[13] E.Y. Jang, et al., Single-walled carbon-nanotube networks on large-area glass

substrate by the dip-coating method, Small 4 (12) (2008) 2255–2261.[14] F. Mirri, et al., High-performance carbon nanotube transparent conductive

films by scalable dip coating, ACS Nano 6 (11) (2012) 9737–9744.[15] R.C. Tenent, et al., Ultrasmooth, large-area, high-uniformity, conductive

transparent single-walled-carbon-nanotube films for photovoltaics producedby ultrasonic spraying, Adv. Mater. 21 (31) (2009) 3210.

[16] Q.F. Liu, et al., Free-standing highly conductive transparent ultrathin single-walled carbon nanotube films, J. Am. Chem. Soc. 132 (46) (2010) 16581–16586.

[17] A.R. Boccaccini, et al., Electrophoretic deposition of carbon nanotubes, Carbon44 (15) (2006) 3149–3160.

[18] A. Sarkar, T. Daniels-Race, Electrophoretic deposition of carbon nanotubes on3-amino-propyl-triethoxysilane (APTES) surface functionalized silicon sub-strates, Nanomaterials 3 (2) (2013) 272–288.

[19] D. Mann, et al., Ballistic transport in metallic nanotubes with reliable Pd ohmiccontacts, Nano Lett. 3 (11) (2003) 1541–1544.

[20] S.C. Tsang, et al., A simple chemical method of opening and filling carbonnanotubes, Nature 372 (6502) (1994) 159–162.

[21] B.H. Chen, et al., Complementary carbon nanotube-gated carbon nanotubethin-film transistor, Appl. Phys. Lett. 88 (9) (2006) 093502.

[22] C.X. Chen, W. Zhang, Y.F. Zhang, Multichannel carbon nanotube field-effecttransistors with compound channel layer, Appl. Phys. Lett. 95 (19) (2009)192110.