Controllable carrier density of pentacene field-effect transistors using polyacrylates as gate dielectrics

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Controllable carrier density of pentacene ﬁeld-effect transistors

using polyacrylates as gate dielectrics

Jung-An Cheng

*

_{, Chiao-Shun Chuang, Ming-Nung Chang, Yan-Chu Tsai, Han-Ping D. Shieh}

Department of Photonics and Display Institute, National Chiao Tung University, Rm. 501 CPT Building, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan

a r t i c l e

i n f o

Article history: Received 2 May 2008

Received in revised form 5 August 2008 Accepted 11 August 2008

Available online 19 August 2008

PACS: 52.25.Mq 61.66.Hq 61.82.Fk 61.82.Ms 61.82.Pv 68.47.Mn Keywords: OTFT Polyacrylate Pentacene PMMA Dielectric

a b s t r a c t

We have studied the effect of the chemical structure of dielectrics by evaporating penta-cene onto a series of polyacrylates: poly(methylmethacrylate), poly(4-methoxyphenylac-rylate), poly(phenylacpoly(4-methoxyphenylac-rylate), and poly(2,2,2-trifluoroethyl methacrylate) in organic thin-film transistors (OTFTs). In top-contact OTFTs, the polyacrylates had a significant effect on field-effect mobilities ranging 0.093 0.195 cm2_V1_s1_{. This variation neither} corre-lated with the polymer surface morphology nor the observed pentacene crystallite size. This result implies that the PTFMA device generates the local electric field that accumulates holes and significantly shifts the threshold voltage and the turn-on voltage to 8.62 V and 3.5 V, respectively, in comparison with those of PMMA devices.

1. Introduction

Ever since the first organic thin-film transistors (OTFTs) based on polymer[1–3]and small molecule[4,5] semicon-ductors were reported, interest in this field has risen stea-dily for both technological and scientific reasons. OTFTs are of interest for a variety of electronic applications, such as radio frequency identification (RFID) tags[6], flexible dis-play[6,7], sensors[8], and electronic barcodes[9,10]. Pro-totypes of these products have been demonstrated and are now gearing towards commercialization.

Recent technological advances in OTFTs have triggered intensive research into the molecular and mesoscale

struc-tures of organic semiconductor films that determine their charge transport characteristics. Since the molecular struc-ture and morphology of an organic semiconductor are lar-gely determined by the properties of the interface between the organic film and dielectric, a great deal of research has focused on interface engineering. Kobayashi et al.[11], have reported that self-assembled monolayers (SAMs) accumu-late holes in the transistor channel. These properties can be understood in terms of the effects of the electric dipoles of SAM molecules, and weak charge transfer between organ-ic films and SAMs. The technique provides a simple way of controlling channel charge density and thus also the thresh-old voltage at low density levels, which should be of useful for fabricating OTFTs with improved functionality[12].

Polymeric insulators have been considered as prefera-ble gate dielectric materials, in part, because ﬁlms exhibit-ing good characteristics can often be formed simply by

*Corresponding author. Tel.: +886 3 5712121x59210; fax: +886 3 5737681.

E-mail address:[email protected](J.-A. Cheng).

Contents lists available atScienceDirect

Organic Electronics

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spin-coating, casting, or printing at room temperature and under ambient conditions. The mobility of the most penta-cene-based OTFTs is signiﬁcantly inﬂuenced and domi-nated by characteristics of applied gate dielectrics. PMMA is a well-known glasslike material in photonics[13]. Re-cently, PMMA used as a gate dielectric layer in OTFTs is also being considered, because of it shows not only good output and transfer characteristics but also potential to alternate SiO2as the gate dielectric in OTFTs[14–17].

In this paper, we present the study of the field-effect on pentacene-based organic TFT by using polyacrylates as dielectric layers. To control channel conductance and car-rier density without using gate voltage, a series of dipole tunable polyacrylates, which are derived from PMMA matrix, are investigated as dielectric layers. By using penta-cene-based OTFTs with a top-contact configuration, we will also characterize the dipole effect on the semiconductor/ dielectric interface. The output and transfer characteristics related to carrier density, including field-effect mobility (

lsat

), on-off current ratio (Ion/Ioff), turn-on voltages (Von),

and threshold voltages (VTh), were also examined.

2. Experimental

All of the materials used in the experiment were com-mercially available, and solvents were used without fur-ther puriﬁcation. The purity of gold and aluminum metal are 99.8%. The molecular weight of commercial poly(meth-ylmethacrylate) (PMMA) is 15,000 g/mol. Poly(phenylacry-late)[18], poly(4-methoxyphenylacrylate) (PMPA)[19], and poly(2,2,2-triﬂuoroethyl methacrylate) (PTFMA)[20]were prepared according to the references. Thermal decomposi-tion temperature (Td), glass transition temperature (Tg),

and molecular weight (Mw) of polyacrylates were

deter-mined by thermogravimetric analysis (TGA, SEIKO I TG/ DTA 200), differential scanning calorimetry (DSC, SEIKO SII DSC 2000), and a Water 600 gel permeation chromatog-raphy (GPC), respectively. Surface afﬁnity of polyacrylates thin-ﬁlms was measured by using a Paul N. Gardner con-tact angle detector.

2.1. Device fabrication

The patterned ITO glass substrates were cleaned ultra-sonically with detergent, deionized water, 2-propanol, and methanol, followed by UV-ozone pretreatment before use. For top-contact OTFTs fabrication, polyacrylates were dissolved in toluene with 12.0 wt%, spin-coated onto ITO substrates at 1000 rpm for 30 s, and dried in a vacuum oven at 80 °C for 4 h. Thereafter, pentacene was thermally deposited at 6 10–6Torr (ca. 0.3–0.4 Å s1_{) with a ﬁlm}

thickness of 600 Å. Top-contact Au electrodes (ca. 70 nm) were then deposited through a shadow mask. The channel length (L) and width (W) of the devices were 200 and 2000

l

m, respectively. The ﬁlm thickness and roughness were measured using a DI 3100 atomic force microscopy (AFM). The current–voltage (I–V) characteristics of OTFTs were measured using a Keithley 4200 semiconductor parameter analyzer and a HP 4284 CV analyzer.

3. Result and discussion

3.1. Characterization of gate dielectric surface

To investigate the effect of polyacrylates gate dielectrics on pentacence-based OTFTs, a series of polyacrylates with different ester substitutes (4-methoyxphenyl (PMPA), phe-nyl (PPA), and 2,2,2-triﬂuoroethyl (PTFMA) side chain

* CH3 * O O n CF3 * H * O O n O C H3 * H * O O n * CH3 * O O CH3 n Glass Substrate ITO polyacrylate Source Drain Au Au Pentacene

Pentacene

PMMA PMPA PPA PTFMA

a

Fig. 1. (a) The schematic of top-contact OTFT and (b) chemical structures of materials used in this study.

Table 1

Gate dielectric thin-ﬁlm surface characteristics Dielectric material Tg(°C) Thickness (nm) Surface roughness (Ra; nm)

Surface contact angle (°)a

Surface free energy (SFE; mJ m2₎ Dielectric constant (k)b PMMA 105.2 560 0.32 74 38.19 3.2 PMPA 133.2 660 0.40 75 37.54 3.4 PPA 135.7 582 0.20 90 27.91 2.9 PTFMA 82.4 592 0.44 95 24.75 6.0 a

Measured by using DI-water droplets.

b

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groups) were synthesized. Their corresponding chemical structures and the OTFT conﬁguration are depicted in

Fig. 1.

The dielectric thermal properties and thin-ﬁlm surface were characterized and summarized inTable 1. In thermal analysis, both PMPA and PPA show higher Tgthan that of

PMMA and PTFMA, implying the rigidity of the side-chain group. All dielectrics had comparable thickness (560660 nm) were spin-cast, and exhibited very similar topologies. The deposited dielectric thin-ﬁlms were ob-served to be pinhole-free and smooth; whose root-mean-square roughness was within 0.20–0.44 nm.

Improved mobility could be depicted by changes in the surface energy of gate dielectrics[21]. In addition, lower-ing the surface energy on a gate dielectric was identiﬁed as a key factor to increasing the grain size during the growth of pentacene. To evaluate those parameters related to the thin-ﬁlm surface free energy (SFE), we utilize Eq.(1), obtained from both Young’s equation and the equation of

state for solid/liquid interfacial tension, to estimate the surface free energy (SFE) of gate dielectrics[22].

c

Lð1 þ coshÞ ¼ 2ð

c

L

c

sÞ 1=2

ebðcLcsÞ2_; _ð1Þ

where

cL

,

cs

, h, and b is the surface tension of water, the surface energy of the solid, the measured contact angle, and an empirical constant with an average value of 1.06 104_(m2_mJ1₎2_{, respectively. In estimating surface}

energy

cs

, the surface tension of water is substantially adopted as 72.88 mJ/m2_.

The surface contact angle of polyacrylates and their cor-responding SFE, which were calculated according to Eq.(1), summarized inTable 1, are dominated by the hydropho-bicity of ester groups SFE also decreases with increasing the surface contact angle. In structural modiﬁcation, dielectric constant (k) is substantially increased for PMPA and PTFMA while high polar substitutes instead of methyl groups in ester linkage. The dielectric constants of PPA, PMMA, PMPA, and PTFMA were obtained as 2.9, 3.2, 3.4,

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and 6.0, respectively. The measured k value of PTFMA is ex-tremely high, owing to the electronegative characteristic of the 2,2,2-triﬂuoroethyl group.

3.2. Topography of pentacene thin-ﬁlm

To study thin-film growth mechanisms during the ther-mal evaporation process, we utilized the AFM to measure the surface morphology of pentacenes of 2, 30, 60, and 100 nm in thickness. The profiles were measured in the channel region and are shown inFig. 2, where grain size in-creases with film thickness, the grain shape is independent of pentacene film thickness. Although the surface mor-phology of pentacene is measured on different polyacry-late dielectrics, the typical crystallites size is remarkably similar, roughly 0.3–0.7

l

m, as shown inFig. 2. This fact is also found that the pentacene ﬁlm growth mode/mor-phology variations are closely correlated with the surface energy of the corresponding polymer substrates. Different semiconductor ﬁlm growth mechanisms are involved with different dielectric substrates, which can be associated with either formal Frank-van der Merwe (layer-by-layer) or Volmer-Weber (island) growth modes[23]. The

nee-dle-like growths, visible in the images of PMMA, PMPA, and PPA are most likely pentacene dihydride [24]. Most of the substrates show crystallites of the usual dendritic form, and their corresponding pentacene growth mecha-nisms show as layer-by-layer growth modes. However, the growth mechanism of PTFMA is totally different from that of mentioned above. The dendritic feature is not ob-served in PTFMA substrate. What displaces are dense grains with island growth mode, implying that the crystal growth of the first seeding of pentacene on the flat sub-strate was significantly affected when the dielectric surface energy relatively approaches to that of surface energy on pentacene crystal plane[25,26].

3.3. The output and transfer characteristics of OTFTs The output characteristics (IDSvs. VDS) with various gate

voltages (VGS) for pentacene TFTs using polyacrylates as

the gate insulator are plotted inFig. 3. All devices show good saturation characteristics as a drain-source bias. The IDS–VDSoutput characteristics show a p-channel

accumula-tion type ﬁeld-effect transistor. Under a given gate voltage VGS, the device with PTFMA gate dielectric shows similar

-30 -25 -20 -15 -10 -5 0 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -30 V -20 V -10 V V_GS = 0 V D rain-source Current I DS (uA ) Drain-source Voltage V_DS (V) -30 -25 -20 -15 -10 -5 0 -0.6 -0.4 -0.2 0.0 -30 V -20 V -10 V V_GS = 0 V Dr ai n-sour ce Cur rent I DS (uA ) Drain-source voltage V_DS (V) -30 -25 -20 -15 -10 -5 0 -1.5 -1.0 -0.5 0.0 -30 V -20 V -10 V V_GS = 0 V Drai n-source Cur rent IDS (u A ) Drain-source Voltage V DS (V) -30 -25 -20 -15 -10 -5 0 -1.5 -1.0 -0.5 0.0 -30 V -20 V -10 V V_GS = 0 V Dr ai n-sour ce Curr ent I DS (uA ) Drain-source Voltage V_DS (V)

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features of drain-source current curves with PMMA gate dielectrics, and these devices mentioned above show high-er saturation IDSthan that of PMPA and PPA at VGS= 30 V.

The transfer characteristics of the devices are measured in the saturation region (|VDS| = 30 V P |VGS–VTh|), and

rep-resentative transfer plots are shown inFig. 4. The carrier mobility (

lsat

) and threshold voltage (VT) are calculated

from the slop and intercept of the linear part of the (IDS1/2VG) plot by ﬁtting the data to Eq.(2) [27]:

l

sat¼ ð2IDSLÞ=½WCiðVG VThÞ; ð2Þ

where IDS, L, W, Ci, VG, and VThare the drain saturation

cur-rent, channel length, channel width, gate dielectric capac-itance, gate voltage, and threshold voltage, respectively. All parameters related to transfer characteristics are sum-marized inTable 2.

In this study, the observed charge-carrier mobility de-pends not only on the dielectric surface chemical charac-teristics but also on the dipole moment of the terminal groups on the dielectric side-chain. As shown inTable 2, gate insulators, with low surface energy and high dielectric constants, show a tendency to have high mobility due to the uniform pentacene thin-ﬁlm phase and good crystallo-graphic structure. Accordingly, PMMA and PMPA have sim-ilar dielectric constants and surface free energies. They also show similar topography with layer-by-layer crystal grow-ing mode in AFM images (Fig. 2). Furthermore, their corre-sponding ﬁeld-effect mobilities at the saturation region (

lsat

) are similar and measured at 0.153 cm2_V1_s1 _for

PMMA and 0.134 cm2V1_s1_{for PMPA. In general,}

penta-cene ﬁlms grown in the layer-by-layer mode exhibit large grain sizes and relatively similar mobilities (e.g., PMMA and PMPA), but smaller grain sizes and poor mobilities in the island mode. However, pentacene ﬁlm with the island mode growth in PTFMA device affords different OTFT char-acteristics (

lsat

= 0.195 cm2_V1_s1_{) from that of the}

layer-by-layer mode. Thus, the field-effect mobility is relatively dropped when the thin-film phase and the single crystal phase synchronously grow and coexisted during thermal evaporation.[28]. Evidently, the topographies of PTFMA show the single crystal phase inFig. 2. Hence, the PTFMA device has higher field-effect mobility than that of PMMA and PMPA.

The device characteristics of pentacene TFT based on different gate dielectrics are shown inFig. 4a; the source-drain current IDSin a logarithmic scale are shown against

the gate voltage VGat a constant source-drain voltage of

VDS= 30 V. All of these devices exhibit Ion/Ioffvalues of

four orders of magnitude and also show that IDSvalues

de-pend on the nature of polyacrylates. The IDS value at

VG= 0 V is enhanced by one order of magnitude in devices

with PTFMA compared with that of PMMA, indicating that

10-11 1x10-10 1x10-9 1x10-8 1x10-7 1x10-6

PMMA

PMPA

PPA

PTFMA

Dr

ai

n Cur

rent

I

DS

(A

)

-30 -20 -10 0 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

PMMA

PMPA

PPA

PTFMA

Gate Voltage V_G (V)

I

DS 1/ 2

(m

A

)

1/2

Fig. 4. Transfer characteristics of OTFTs with various gate dielectric materials. The gate voltage is swept at a constant drain-source voltage VDS= 30 V.

Table 2

Summary of the electrical performance parameter for OTFTs with different gate dielectrics Dielectric Mobility (lsatcm2V1s1) Capacitance (Ci; nF cm2) On–off current ratio (Ion/Ioff) Turn-on voltage (Vto; V) Threshold voltage (VTh; V) Dipole moment (D; Debye)[30] PMMA 0.153 5.06 6.64 104 0.0 7.03 0.94a PMPA 0.134 4.56 4.72 104 _1.0 6.03 1.36b PPA 0.093 4.41 3.97 104 _1.0 8.24 1.16c PTFMA 0.195 5.35 2.83 104 3.5 8.62 2.54d

Note: the side-chain group.

a

Alkyl group (–OCH3). b

Alkoxyaryl (–OC6H5OCH3). c

Phenyl group (–OC6H5). d _{Fluoroalkyl group (–OCH}

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the surface carrier is modulated by changing the terminal ester group on gate dielectrics.

The I1=2

DS–VGrelations for devices are plotted inFig. 4b.

Through a linear ﬁt for I1=2_DS–VGcurves, we are able to

esti-mate VTh and

lsat

in the saturation region. Of particular

interest is the change in VTh with various polyacrylates.

VThand Vtoshift positive as dielectrics go from PPA through

PMMA to PTFMA.

The observed VThand Vtoshift correlates to the electron

affinity of the polymeric insulators end group. The dipole structures synchronously form a built-in dipole-dipole field, and the induced field is equivalent to a gate voltage applied in the transistor channel. The electronegativity of the terminal functional group on polymeric dielectrics influences the charge distribution within the gate insulator and pentacene. This can lead to the formation of an electric dipole within the dielectric/pentacene interface. The field induced phenomenon was also studied by Campbell et al.

[29], who proposed the charge distribution within similar molecules and found a dipole moment whose strength sig-nificantly depended on the functional group of the investi-gated molecules. In this presented circumstance, the electron affinity of a close-packed organized organic monolayer can differ from the properties of the isolated molecules. As a result, the charge density at the insula-tor-organic semiconductor interface has been organized in advance so that it would affect device performance dra-matically. The change in surface potentially modifies inter-face properties, as illustrated in the schematic band diagram shown in Fig. 5. When pentacene is deposited onto a dielectric layer without the dipole effect, the vac-uum levels are aligned and no bending of the HOMO and LUMO level occurs, as shown inFig. 5a. When a negative gate voltage is applied, the Fermi level of the gate electrode shifts towards higher (electron) energies. A part of the ap-plied gate voltage is dropped across the gate insulator, and since the band alignment of the HOMO and LUMO level is fixed with respect to the vacuum level, the remaining gate voltage bends HOMO and LUMO levels. As a result, mobile charge carriers can be accumulated and formed in the con-ducting channel. For a polymeric dielectric with a perma-nent dipole field inserted between the gate electrode and the pentacene, as shown inFig. 5b, the dipole field of the polymeric insulator modifies the surface potential which has the same effect as applying a (negative) gate voltage. For a p-type OTFT, therefore, VThand Vtowill shift to more

a positive region when a polymer with high dipole mo-ment is used as a gate dielectric layer.

4. Conclusion

We have demonstrated that the single-layered polyac-rylate with electronegative side groups significantly in-duces a built-in field within the semiconductor/ dielectrics interface in p-type organic TFTs. This built-in field also shifts both threshold voltage (VTh) and turn-on

voltage (Vto) to positive bias voltage in pentacene-based

devices with increasing the dipole moment of the dielec-tric. Although SAMs approach was also employed as gate dielectrics and the ultrathin monolayer less than 10 nm would be effective for reducing the operational voltage, it would be a technical challenge for making a pin-hole free monolayer ﬁlm in a large area. Accordingly, we ﬁnd that our study provides a simple way of controlling channel charge density at very low density levels and consequently the VThvalue, which should be useful for fabricating OTFTs

with improved functionality. Permittivity tunable poly-meric dielectrics are also a promising route to alternative SAM processed, and they will also open up exciting possi-bilities for fabrication of large area TFT devices by inkjet printing.

Acknowledgement

This work was supported by the MOE ATU Program ‘‘Aim for the Top University” # 97W802 and NSC-96-2628-E009-021-MY3.

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Vacuum Level

Ener

gy

Gate Insulator Pentacene

HOMO Ener gy Pentacene GateInsulator Dipole Effect Region - + + -LUMO Fermi level

Fig. 5. Schematic energy level diagram for illustrating the interface between gate insulators and pentacene: (a) without and (b) with dipole effect between gate dielectric/pentacene interface. The ‘‘+” and ‘‘” denote the hole and the electron, respectively.

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