Abstract
The authors examine the effects of adsorption of four thiolated molecules (HS- C10 H21, HS- C11 H22 OH, HS- C10 H20 COOH, and HS- C2 H4 C4 F9) on the electrical characteristics of single-walled carbon nanotube network FETs (SNFETs). Work function of the electrodes was measured before and after molecule adsorption. Schottky barrier energy extraction for SNFETs was also performed and the results provide direct evidence that the device characteristics of SNFETs after SAM adsorption are altered primarily due to the change in energy-level alignment between the Au and SWNTs, which thus provides an effective methodology for the tuning and performance optimization of these devices in a controllable way.
| Original language | English |
|---|---|
| Article number | 103515 |
| Journal | Applied Physics Letters |
| Volume | 91 |
| Issue number | 10 |
| DOIs | |
| Publication status | Published - 2007 |
| Externally published | Yes |
All Science Journal Classification (ASJC) codes
- Physics and Astronomy (miscellaneous)
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Tuning of electrical characteristics in networked carbon nanotube field-effect transistors using thiolated molecules. / Lee, Chun Wei; Zhang, Keke; Tantang, H. et al.
In: Applied Physics Letters, Vol. 91, No. 10, 103515, 2007.Research output: Contribution to journal › Article › peer-review
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TY - JOUR
T1 - Tuning of electrical characteristics in networked carbon nanotube field-effect transistors using thiolated molecules
AU - Lee, Chun Wei
AU - Zhang, Keke
AU - Tantang, H.
AU - Lohani, Anup
AU - Mhaisalkar, S. G.
AU - Li, Lain Jong
AU - Nagahiro, T.
AU - Tamada, K.
AU - Chen, Y.
N1 - Funding Information: Lee Chun Wei Zhang Keke Tantang H. Lohani Anup Mhaisalkar S. G. Li Lain-Jong a) School of Materials Science and Engineering, Nanyang Technological University , 50 Nanyang Avenue, Singapore 639798, Singapore Nagahiro T. Tamada K. Department of Electronic Chemistry, Tokyo Institute of Technology , Kanagawa 226-8502, Japan Chen Y. School of Chemical and Biomolecular Engineering, Nanyang Technological University , 50 Nanyang Avenue, Singapore 637819, Singapore a) Electronic mail: ljli@ntu.edu.sg 03 09 2007 91 10 103515 30 05 2007 25 07 2007 07 09 2007 2007-09-07T11:15:46 2007 American Institute of Physics 0003-6951/2007/91(10)/103515/3/ $23.00 The authors examine the effects of adsorption of four thiolated molecules ( H S – C 10 H 21 , H S – C 11 H 22 O H , H S – C 10 H 20 C O O H , and H S – C 2 H 4 C 4 F 9 ) on the electrical characteristics of single-walled carbon nanotube network FETs (SNFETs). Work function of the electrodes was measured before and after molecule adsorption. Schottky barrier energy extraction for SNFETs was also performed and the results provide direct evidence that the device characteristics of SNFETs after SAM adsorption are altered primarily due to the change in energy-level alignment between the Au and SWNTs, which thus provides an effective methodology for the tuning and performance optimization of these devices in a controllable way. Carbon nanotubes (CNTs) have shown great promise in nanoscale field-effect transistors (FETs) since they can serve as high-mobility transport channels. 1 However, precise position and alignment of CNTs is still not available in current growth and assembly technology. Therefore, there is a growing body of research on the development of carbon nanotubes network FETs. 2,3 Electrical detection of deoxyribonucleic acid and biomolecules using carbon nanotubes network FETs have been demonstrated. 4,5 Such devices have been also found to be sensitive to various gases and thus can be used as chemical sensors. 6 The sensing behaviors of these network devices strongly depend on the electrical characteristics in their as-produced state. Therefore, optimizing the performance of these devices requires a better understanding of the method for adjusting their electrical characteristics prior to further applications in electronics and molecular detection. It has been reported that single-walled CNTs (SWNT)-FETs operate as Schottky barrier (SB) transistors, 7 where the switching occurs primarily by modulation of the contact resistance rather than the channel conductance. Alkanethiols and their derivatives are known to form self-assembled monolayers (SAMs) on Au and subsequent work functions modulation via dipole formation on the metal surfaces have been reported. 8 The hole-injection processes across the metal/organic semiconductor interface are thought to be affected by SAM formation on the electrodes, 9 suggesting that the alkanethiol interfacial dipole layer influences the energy-level alignments at the CNT/Au contacts. 10,11 Alkanethiol modifications have been reported to reduce drain current 10 whereas a dodecanethiol has been reported to have a negligible effect on the same. 11 Therefore, the drain current I d and SB modulation in SWNT-FETs warrants further studies to elucidate whether the dipole from the molecules near the Au-CNT junction can adjust the SB energy of the FETs. The results provide direct evidence that the device characteristics of single-walled carbon nanotube network FETs (SNFETs) after SAM adsorption are altered primarily due to the change in energy-level alignment between the Au and SWNTs. The devices used in our experiments were in a bottom contact configuration [Fig. 1(a) ], where the Au electrodes (channel length was 140 μ m ) were directly sputtered on a p -type low resistance Si wafer with a thermal oxide layer ( 80 nm ) on it. SWNT (arc discharge produced tubes 12 ) suspensions were then drop cast on the devices, followed by rinsing of de-ionized water and 2 h of annealing at 200 ° C in air. Prior to ancamethrol modification, these devices were further rinsed with ethanol and de-ionized water. The self-assembly was performed by immersing the devices into an ∼ 1 mM alkamethrol solution in ethanol for 16 h , followed by rinsing of ethanol. The two atomic force microscopy images in Fig. 1 show the CNT networks on the channel area and the Au electrode surface. The transfer curves ( I d as a function of applied gate voltage V g ) for the devices before and after modification with decanethiol ( H S – C 10 H 21 ) and 1 H , 1 H , 2 H , 2 H -perfluoro-1-hexanethiol ( H S – C 2 H 4 C 4 F 9 ) are shown in Figs. 1(b) and 1(c) , respectively. Figure 1(b) displays a significant decrease in I d after decanethiol modification as well as a significant change in the transconductance [the I d - V g slope, as indicated in Fig. 1(b) ]. This change in transconductance could be attributed to (i) the increase of carrier scattering centers (in the channel area) through molecule adsorption or (ii) the increase in the barrier energy at the Au/CNT interface. The first possibility was excluded by means of x-ray photomission spectroscopy (XPS) studies, where XPS data confirmed the absence of any thiolated moieties on the CNT ∕ Si O 2 substrates. By contrast, pronounced peaks due to the sulfur S 2 p (binding energies: 162.2 and 163.4 eV ) were observed on the CNT/Au samples. This observation indicates that there is no detectable decanethiol in the channel area of the devices. Therefore, (i) was excluded for the explanations of the reduction in I d after decanethiol modulation. The threshold voltage ( V th ) after the decanethiol modification [Fig. 1(b) ] is slightly shifted to negative V g , indicating that the electronic doping (to CNTs) is another possible explanation for the decrease in I d . In principle, the effect of electronic doping will purely shift the I d - V g curve without changing its slope (transconductance). Therefore, in addition to the effect of electronic doping, the modulation of Au/CNT interface may also be responsible for the decrease in I d after decanethiol modification. By contrast, the SNFET modified by H S – C 2 H 4 C 4 F 9 shows an increase in I d , [Fig. 1(c) ]. The V th is not obviously shifted whereas the transconductance is significantly increased after modification, suggesting that the contact resistance between Au and CNT is lowered (or SB energy is lowered) and may be mainly responsible for the I d increase. The effect of adsorption of mercaptoundecanol ( H S – C 11 H 22 O H ) and mercaptodecanoic acid ( H S – C 10 H 20 C O O H ) was also investigated, and a relatively smaller change in drain current was observed (Table I ). The operational mechanism of carrier injection from Au electrodes into SNFETs has been reported same as individual SWNT-FET, 13 where the SB plays a dominant role. Tunneling does not play an important role at the temperature higher than 20 K for CNT transistors 13 and the metal-CNT barrier height has been extracted using Arrhenius equation I d ∼ exp [ − E a ∕ kT ] , 13,14 where E a is the activation energy (estimated barrier energy), k is the Boltzmann constant, and T is the temperature. The SB energy extraction for two extreme cases (SNFETs modified by decanethiol and H S – C 2 H 4 C 4 F 9 ) was performed following a methodology reported previously in the literature. 14 Figure 2(a) shows the Arrhenius plot ( I d vs 1 ∕ T ) for a H S – C 2 H 4 C 4 F 9 modified SNFET at various V g (5, 10, 15, 20, 23, and 25 V ) in vacuum ( 10 − 7 torr ) , where the data are linearly fitted at the temperature range from 200 to 300 K at applied drain voltage V d = − 0.05 V [Fig. 2(a) ]. Extracted activation energies ( E a ) at various V g for a bare SNFET and the SNFETs modified with decanethiol and H S – C 2 H 4 C 4 F 9 are displayed in Fig. 2(b) . The SB energy for each device is obtained from the E a of the peak position, which is actually in the depletion region where the band is flat. The SB energy extracted for the bare CNT is around 145 meV , close to the reported value of 170 meV for a Au-CNT contact. 13 The SB energy of the bare SNFET was increased by 34 meV after decanethiol modification, whereas it was reduced by 89 meV after H S – C 2 H 4 C 4 F 9 modification, which further corroborates the theory that the adsorption of thiolated molecules leads to the barrier energy change. The SB data were further compared with contact potential difference (CPD) measurements using a Kelvin probe on Au and Au substrates absorbed with four thiolated molecules (Table I ). The trend of work function change of Au ( Δ Φ ) we obtained is consistent with the reported data. 15 The SB energy and the change in I d ( Δ I d , measured at V g = − 5 V , V d = 10 V ) of the SNFETs due to the modification of various thiolated molecules are also shown in the Table I . The change in I d is consistent with the Δ Φ , i.e., decanethiol modification case, the negative Δ Φ results in a higher SB energy and therefore lower I d in the SNFETs, suggesting that the electrical characteristics can be tuned by adjusting barrier energy through the modulation of the energy levels between Au and SNFETs using thiolated molecules. The relative energy-level lineup for CNT and Au electrodes before and after modification is schematically illustrated in Fig. 3(a) . This simplified picture aids the explanation of I d change in a first approximation. Furthermore, the change in SB energy is consistent with the change in work function of Au, indicating that the SB tuning is related to the dipole of the thiolated molecules although the change in SB is smaller than the change in Au work function. This observation is consistent with the report by Lang and Avouris, 16 where they suggest that the short range dipole (or electrostatic) interaction near the actual Au-CNT junction may interact with the junction and hence influence the energy level alignments. Figure 3(b) illustrates the structure of the Au-CNT contact point and also the dipole direction formed between Au and the absorbed thiolated molecules 17 (only the two extreme cases are shown: decanthiol and H S – C 2 H 4 C 4 F 9 ). The obtained data suggest that the dipoles formed adjacent to the Au-CNT contact are the contributing factor for the change in the SB energy. In summary, we have demonstrated that the junction barrier energy between Au and CNT can be adjusted by various thiolated molecules. The change in electrical characteristics ( I d ) follows the change in work function of Au, suggesting that it is possible to alter or optimize the electrical behaviors of SNFETs through modifications of Au electrodes with suitable thiolated molecules. Energy level diagrams based on the Au contact potential and SB measurements may be effectively used to provide a first approximation for the change in drain current. The barrier energy modulation is influenced by the interaction between the dipoles formed adjacent to the Au-CNT contact, where the short range dipole or electrostatic interaction may interact with the junction and hence alter the energy level alignment between CNT and Au electrodes. In addition to energy level alignment, the role of oxygen doping in ambient is still unclear and requires more studies. Further theoretical studies are required to understand the details of the short range dipole interaction at the carbon nanotube-Au junction. The methodology described herein could play an important role in the optimization of sensors, photovoltaic, as well as light emitting transistor and diode applications. This research was supported by Nanyang Technological University, Singapore. Table I. Comparison of work function of Au electrodes and electrical characteristics of SNFET devices modified with the four thiolated molecules. Sample a CPD b (V) Δ Φ c (mV) Δ SB d (meV) Δ I d e (%) Au − 0.600 0 ⋯ H S – C 10 H 21 ∕ Au − 0.089 − 511 + 34 − 61.1 H S – C 11 H 22 O H ∕ Au − 0.503 − 97 ⋯ − 34.3 H S C 10 H 20 C O O H ∕ Au − 0.645 + 45 ⋯ + 13.6 H S – C 2 H 4 C 4 F 9 ∕ Au − 1.205 + 605 − 89 + 122.4 a Samples for CPD (and Δ Φ ) measurements were plain Au and Au modified with thiolated molecules indicated. Samples for Δ SB and Δ I d were SNFETs and those modified with thiolated molecules as indicated. b CPD: contact potential difference, defined as the vacuum-level difference. c Δ Φ : change in CPD relative to Au. Negative sign represents a lower work function (higher energy level). d Δ SB : Schottky barrier energy change of the SNFETs after modification relative to a bare SNFET. e Δ I d : increase of drain current measured at V g = − 5 V and V d = + 10 V . Data represent an average of three samples with a typical standard deviation of less than 15%. FIG. 1. (Color online) (a) Schematic device structure of SNFETs. Atomic force microscopy images show the networked CNT structures on channel and Au, respectively. [(b) and (c)] Transfer characteristics for devices in ambient before and after modification with (b) decanethiol showing a decrease in I d and (c) 1 H , 1 H , 2 H , 2 H -perfluoro-1-hexanethiol, showing an increase in I d . Inset shows the corresponding device output characteristics ( I d - V d ) . FIG. 2. (Color online) (a) Arrhenius plot ( I d vs 1 ∕ T ) for a 1 H , 1 H , 2 H , 2 H -perfluoro-1-hexanethiol modified SNFET at various V g (5, 10 15, 20, 23, and 25 V ) in vacuum. The V d is − 0.05 V . (b) The extracted SB energies for a bare SNFET and the SNFETs modified with decanethiol and 1 H , 1 H , 2 H , 2 H -perfluoro-1-hexanethiol, showing an increase in SB for the former and a reduction in SB energy for the latter. FIG. 3. (Color online) (a) Relative energy-level lineup for CNT and Au electrodes before and after modification. (b) The structure of the Au-CNT contact point and also the dipole direction (indicated by arrows) formed between Au and the absorbed thiolated molecules.
PY - 2007
Y1 - 2007
N2 - The authors examine the effects of adsorption of four thiolated molecules (HS- C10 H21, HS- C11 H22 OH, HS- C10 H20 COOH, and HS- C2 H4 C4 F9) on the electrical characteristics of single-walled carbon nanotube network FETs (SNFETs). Work function of the electrodes was measured before and after molecule adsorption. Schottky barrier energy extraction for SNFETs was also performed and the results provide direct evidence that the device characteristics of SNFETs after SAM adsorption are altered primarily due to the change in energy-level alignment between the Au and SWNTs, which thus provides an effective methodology for the tuning and performance optimization of these devices in a controllable way.
AB - The authors examine the effects of adsorption of four thiolated molecules (HS- C10 H21, HS- C11 H22 OH, HS- C10 H20 COOH, and HS- C2 H4 C4 F9) on the electrical characteristics of single-walled carbon nanotube network FETs (SNFETs). Work function of the electrodes was measured before and after molecule adsorption. Schottky barrier energy extraction for SNFETs was also performed and the results provide direct evidence that the device characteristics of SNFETs after SAM adsorption are altered primarily due to the change in energy-level alignment between the Au and SWNTs, which thus provides an effective methodology for the tuning and performance optimization of these devices in a controllable way.
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UR - http://www.scopus.com/inward/citedby.url?scp=34548478767&partnerID=8YFLogxK
U2 - 10.1063/1.2772181
DO - 10.1063/1.2772181
M3 - Article
AN - SCOPUS:34548478767
SN - 0003-6951
VL - 91
JO - Applied Physics Letters
JF - Applied Physics Letters
IS - 10
M1 - 103515
ER -