Sara A. DiBenedetto et al- Molecular Self-Assembled Monolayers and Multilayers for Organic and...

8/3/2019 Sara A. DiBenedetto et al- Molecular Self-Assembled Monolayers and Multilayers for Organic and Unconventional In

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Antonio Facchetti obtained his Ph.D in Chemical Sciences (University of Milan, Italy) under GiorgioPagani. He carried out postdoctoral research at the University of California (Berkeley, USA) with Prof.Andrew Streitwieser and then at Northwestern University with Prof. Tobin J. Marks. In 2002 he joinedNorthwestern University where he is currently an Associate Professor Adjunct. Dr. Facchettisresearch interests include organic semiconductors and dielectrics for thin-film transistors, molecularelectronics, organic nonlinear optical materials, and organic photovoltaics.

Mark A. Ratner is Morrison Professor of Chemistry and Professor of Materials Science andEngineering at Northwestern University. He received his B.S. degree from Harvard University (1964)and his Ph.D. from Northwestern University (1969) under G. Ludwig Hofacker. He is interested instructure and function at the nanoscale, and theory of chemical processes, and tries to unite structureand function in molecular nanostructures, based on theoretical notions, exemplary calculations, and

(importantly) collaborations. Interest areas are molecular electronics, self-assembly, nonlinearresponse, and exact and approximate theories of quantum dynamics and using nanoscience to attackthe energy problems facing this world.

Tobin J. Marks is Ipatieff Professor of Chemistry and Professor of Materials Science and Engineeringat Northwestern University. He received his B.S. degree from the University of Maryland (1966) andhis Ph.D. degree from MIT (1971). His research interests include mechanistic organometallicchemistry and catalysis, optoelectronics, chemical vapor deposition, and molecular electronics.

Sara A. DiBenedetto is currently pursuing her Ph.D. degree under the supervision of Professors TobinJ. Marks and Mark A. Ratner at Northwestern University. She obtained her undergraduate degree inchemistry at Agnes Scott College, Decatur, Georgia (2004). Her research focuses on computational

and physical chemistry for the development of hybrid organic-inorganic dielectric materials for thinfilm transistors.

408 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 14071433


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components. In top-gate arrangements, the gate lines are simplydeposited on top of the semiconductor layer. The basic equationsdescribing the OTFT drain current are[45]

ISDlin WL

mC VG VT VSD2

VSD (1)

ISD satW

2L

mC VG VT 2 (2)

wherem is the field-effect carrier (electron or hole) mobility of thesemiconductor, Wthe channel width, L the channel length, Cthecapacitance per unit area of the dielectric layer, VT the thresholdvoltage, VSD the drain voltage, and VG the gate voltage. OTFTcurrentvoltage (IV) characteristics are typically investigated intwo regimes, the linear current regime (Eq. 1) initially observed at

low drain voltages (VSD100nmthick) as the gate dielectric. This materialprevents gate leakage currents, acts as aneffective capacitor, and allows for the accuratemeasurement of the electrical performance of

the organic semiconducting layer. The majormotivation to search for alternative gate dielec-tricsis to enable inexpensive fabrication (e.g., byprinting near room temperature) and tosignificantly reduce the OTFT operating vol-tages. According to Equation 1 and 2, a viableapproach to substantially increase the draincurrent, while operating at low biases, is toincrease the capacitance of the dielectric; [46] fora planar structure C "0(kA/d), where k is thedielectric constant, Athe area of the electrodes,and d the dielectric thickness. However,alternative OTFT gate dielectrics should alsohave low gate leakage currents, and be able to

sustain the maximum possible electric dis-placement Dmax"0"E2B, where EB is thedielectric breakdown field.[4749] The observabledielectric parameters, including EB, J (leakagecurrent density), and e (dielectric permittivity)are easily measured in two terminal, planarmetal-insulator-metal(semiconductor) MIM(S)devices.[1]

One of the primary effects of SAMs on theresponse of MIM(S) and OTFT devices the influence of aproximate molecular dipole moment on the semiconductorelectron affinity or on the metal work function. The effect of adipole layer on the surface potential can be estimated from the

Helmholtz equation (Eq. 3).

[50,51]

DV Nm cos u""0

(3)

Here N is the dipole density (cm2), m the dipole moment(debyes, D), uthe average angle the dipole makes with the surfacenormal, and e is the dielectric permittivity. Numerous studieshave documented how SAM dipole moment ultimately mediatesthe charge conduction/injection from the substrate (bottomelectrode)[50,52,53] and the conduction/injection through the SAMand into the semiconductor channel of the OTFT.[5456] While, themain focus of this Review is on SAMs and SAMTs for use as gate

dielectrics in OTFTs, it is clear from the Helmholtz relation (Eq.3) that the interpretation of the electrical properties is complicatedby the intrinsic interplay between the electrostatics of the SAMitself[50] and the electrical interactions between the SAM and theunderlying electrodes/semiconductors,[5761] which will differdepending on the exact TFTcontact geometry. [6264] Despite someof the unresolved aspects of the SAM dielectric-semiconductorinteractions,[65] this Review demonstrates how SAMs and SAMTsare rapidly advancing the field of plastic electronics. Therefore, wesurvey many aspects of SAMs that should be considered in TFTapplications, starting with a description of SAMs and SAMTshaving various chemical structures on metal and oxide surfaces.

Figure 1. A) Common TFT device geometries. B) Examples of typical p-type (group on left), andn-type (group on right) small molecule organic semiconductors.

Adv. Mater. 2009, 21, 14071433 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1409


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2. Fabrication of SAMs and SAMTs

SAMs are ordered molecular assemblies formed by thespontaneous adsorption of an active molecular precursor ontoa solid surface. Usually the precursor molecular species aredissolved in common solvents, however SAMs can be depositedby other techniques, such as vapor deposition, as well. Since there

are detailed descriptions of the chemistries, structures, andcharacterization of various SAMs available in the literature,[6672]

here we only briefly summarize the techniques and molecularstructures of common SAMs and SAMTs.

2.1. SAMs on Metals

The most extensively investigated types of SAMs are alkanethiolson gold,[7380] silver,[8184] copper,[8587] palladium,[88,89] and plati-num[90,91] substrates. However, Au is most commonly used becauseit is easy to deposit as planar thin films and to pattern withconventional lithographic tools or chemical etchants. Further-more, Au is reasonably inert to oxidation and readily binds

organothiols.[92] The most common procedure for SAM deposi-tion is substrate immersion in a dilute solution of the target thiolat room temperature for 1218 h (due to slow reorganizationprocesses during film growth).[93,94] The choice of solvent,solution temperature, concentration, and immersion time areamong the various factors affecting the structure of the resultingSAM.[92,95,96] Examples of thiol (RSH) and dithiol (HSRSH)molecular structures (Fig. 2A) typically deposited on Au forelectronic applications are alkane(di) thiols,[9799] and those basedon oligophenylenes (OPs),[100103] oligo(phenyleneethynylenes)(OPEs), and oligo(phenylenevinylenes) (OPVs),[104109] where thelabile thioacetyl (RSAc) or disulfide (RSSR) functional

groups are sometimes used for substratechemisorption instead of the RSH group.Thiol-derived SAMs with interesting function-alities (Fig. 2B), such as terthiophenes,[110112]

azo-groups,[113117] and tetracyanoquinodi-methane[118,119] have also been investigated.

Alkane(di)thiols form densely packed and well-ordered domains of up to several hundreds ofsquare nanometers on Au,[120] however thenature of the metalsulfur bond and the spatialarrangement of the sulfur groups are stillcontroversial topics. Nevertheless, alkanethiolshave been termed the benchmark for anynew technology related to molecular electronicssince the tunneling electrical properties arerelatively well documented/characterized.[120,121]

2.2. SAMs on Oxides

The use of organosilane precursors (RSiX3, withX Cl, OMe, OEt) to form monolayers requireshydroxylated substrate surfaces, including (butnot limited to) the technologically relevantsurfaces of SiO2, Al2O3, and tin-doped indiumoxide (ITO).[122126] In the case of SiO2 surfaces,the driving force for self-assembly is the in situ

formation of siloxanes, which connect the precursor silane to thesurface silanol (SiOH) groups via very strong SiOSi bonds(Fig. 3).[127] Since the substrate surfaces are amorphous, thepacking and ordering of the chemisorbed organosilanes aredetermined by the underlying siloxane network, by interchaininteractions, and by the reaction temperature.[128] Also, silanes

with particularly short chain lengths and high vapor pressures[such as hexamethyldisilazane (HMDS)] can be deposited onhydroxylated surfaces from the vapor phase by simple exposure tothe molecular vapor at room temperature, or by heating, or byexposure under vacuum.[68,129] Figure 3A illustrates the struc-tures of commonly used SAM silane precursors, such as simplealkane chains octyltrichlorosilane and octadecyltrichlorosilane(OTS and ODTS, respectively), 3-mercaptopropyltrimethoxysilane(MPTMS), hexamethyldisilizane (HMDS), and various types offunctionalized sp molecules.

Other classes of materials deposited on oxide surfaces includen-alkanoic acids (carboxylic end groups, Fig. 3B) and phosphonicacids (Fig. 3C). These classes of molecules have gained attentiondue to their ability to bind to a wide range of metal oxide surfaces

(AgO, Al2O3, ITO) and to form robust SAMs of similar quality tothat of thiols on Au (which may be useful for various technologiesinvolving bottom contact electrodes other than Au). [130,131] In thecase of organophosphonate SAMs, both vapor and solutiondeposition have been demonstrated. Using XPS it was shown thatvapor-phase deposited phenylphosphonic acid (PPOA) reactswith the alumina surface to form POAl bonds. [132] For a seriesof solution deposited phosphonate SAMs, the influence of alkylchain length on deposition was investigated.[129] It was found thatthere is a distinct packing difference between short (C10C13)alkyl and long (C16C18) alkyl phosphate SAMs. For SAMs onTiO2 surfaces, shorter molecules assemble into a less dense,

Figure 2. Some examples of the types of molecular structures used to make SAMs on Au forelectronic applications. A) Mono(di)thiols, and molecular wires (OPE, OPVs, and OPs). B)Molecular wire examples of azo (top), thiophene (middle), cyanoquino alkanedisulfide (bottom)functional groups.



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liquid-like structure as compared to longerchains.[129] The difference is most likely due tostrong bridging bidentate bonding with thesurface for short chains, as opposed to longchains, which are stabilized via the enthalpygain due to van der Waal interactions between

the tightly packed alkyl chains.SAMTs are becoming increasingly impor-tant as gate dielectrics in TFTs (as will bedescribed in Section 4). There are variousmethods for depositing multilayer filmsdescribed in the literature.[133135] Our groupat Northwestern University has been investi-gating a special type of SAMTs, SANDscomposed of alternating layers of s and pconstituent molecules. In addition, multilayersof diphosphonic acids alternating with (andheld together by) Zr4 ions have also beendemonstrated.[68,136] See Figure 3D for exam-ples of general molecular precursor structures

used in SAMTs.The present two-step method of fabricating

SANDs (Fig. 4) involves an iterative combinationof: (i) self-limiting chemisorption of siloxanebuilding blocks, such as a,v-difunctionalizedhydrocarbon chains (Alk), or highly polarizable,siloxy-protected stilbazolium layers (Stb), and(ii) in situ siloxy group removal concurrent withcapping using an octachlorotrisiloxane-derived layer (Cap).[37] This second stepdeposits a robust polysiloxane layer ($0.8nmthick), which is essential for stabilizing/planarizing the molecular layer and regenerat-

ing a reactive hydroxyl surface for subsequentmonolayer deposition.[137] The different typesof multilayers are identified by the combinationof different layers according to the followingnomenclature: Alk Cap (type I), Stb Cap(type II), and Alk Cap Stb Cap (type III).The microstructures and electrical propertiesof IIII have been characterized by specularX-ray reflectivity, standing wave X-ray reflectiv-ity, optical absorption spectroscopy, opticalsecond-harmonic generation measurements,atomic force microscopy (AFM), MIM(S)devices, cyclic voltammetry, and scanningelectron microscopy (SEM).[1]

2.3. SAMs on H-Passivated Si

For certain organic electronic applications(such as high capacitance gate dielectrics) itis desirable to study the direct interfacebetween the organic SAM and Si without theinfluence of the native oxide layer.[70] However,depositing SAMs directly on the Si surfacerequires removing the native oxide, forming areactive surface, and changing the anchoring

Figure 4. Depiction of the molecular reagents used in the growth of SANDs, and fabricationscheme of these components to make the multilayers: types IIII. Reproduced with permissionfrom [1]. Copyright 2005 Wiley-VCH Verlag GmbH.

Figure 3. Examples of molecular structures used for self-assembly on oxide surfaces: A) silanes,B) carboxylic acids, C) phosphonic acids, and D) various precursor structures for SAMTs.



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group of the molecule (since trichlorosilanes do not react cleanlywith the H-passivated Si surface). The reactive Si surface can begenerated either by hydrogen or halogen termination or by

surface reconstruction. The subsequent deposition of the organicSAM is effected by either SiC or SiO bond formation. [70,138]

There are several types of surface reactions to create these types ofSAMs, including: hydrosilylation, thermally induced hydrosilyla-tion, UV-induced hydrosilylation, and chemical grafting usingalcohols or organolithium/organomagnesium reactions.[139,140]

One of the more extensively studied methods uses thefunctionalized reconstructed surface under ultrahigh vacuum(UHV), where molecular precursors with an unsaturated headgroup attach to the HSi surface via a cycloaddition reaction. [141]

Many studies have used the SiH surface to investigate theelectrical properties of SAMs, and a recent review describes thesetypes of systems in more detail.[70]

3. Electrical Characterization of SAMs

SAMs sandwiched between conductive electrodes (see Fig. 5 forvarious test-beds) can be represented by a MIM(S) structure.SAMs of aliphatic chains are expected to be dielectric innature, due to the very large gap between the highest occupiedmolecular orbital (HOMO) and the lowest occupied molecularorbital (LUMO). Utilizing this wide gap, well-ordered SAMsof alkyl chains exhibiting leakage current densities (J)$108106 A cm2 have been successfully integrated in OTFTs

as surface treatments on thick oxides and as gate dielectrics(Section 4.3). SAMs of conjugated molecular structures have alsobeen extensively studied as molecular rectifiers.[142148] Asconjugated SAMs become more widely used in (and on) thegate dielectric of TFTs, it is important to consider theconsequences that their smaller HOMOLUMO gaps may have

on the conduction mechanism of the SAM.

[149]

There is abundantliterature on conduction mechanisms[84,150153] and on chargetransport in SAMs;[70,118,120,144,145,154158] therefore we present asummary of some of the relevant conduction mechanisms andthe most recent results relating to various SAMs.

3.1. Tunneling

Nonresonant tunneling (through bonds) is the most commontransport mechanism observed in molecular SAMs,[159163]

however for p-conjugated molecular SAMs, resonant tunneling(through the molecular orbitals) may also occur due to the smallerHOMOLUMO gap.[164170] The simplest tunneling model

assumes a finite potential barrier at the metalinsulator interfaceand describes the finite probability for electrons to travel a shortdistance into the SAM (or insulator) despite the lack of availableenergy levels. This processes is given by the Simmons relation(Eq. 4), which is expressed here in the simplest form for arectangular barrier to demonstrate the exponential dependence ofthe current density (JDT) on the thickness (d) and barrierheight (f)[70,144]

JDT q2V

h2d2mf1=2 exp 4pd

h2mf1=2

(4)

where q

electron charge, V

applied voltage, h

Plancks

constant, and m electron mass. At low voltages Equation 4can be simplified to JDT / (1/d)exp(bd), where the tunnelingdecay parameter, b (Eq. 5)[171]

b 4p2mfh

a (5)

is accepted to be $0.61 A1 for saturated alkanes, and 0.20.6A1 for conjugated molecules, and smaller b indicates more

efficient tunneling.[145] Here a is a unitless parameter describing

asymmetry of the potential profile (a 1 for a rectangularbarrier). For very large applied voltages (V>f) the barrier

changes to a triangular shape, and the tunneling current is givenby the FowlerNordheim equation (Eq. 6).[70,172175]

JFN q3E2

8phfFNexp

4 ffiffiffiffiffiffiffiffiffi2mp3qhE

qfFN 3=2

(6)

Here fFN is the tunneling barrier height, E the electric field(V/d), and m* is the effective electron mass. FowlerNordheimemission has the strongest dependence on the applied voltage,but (like Eq. 4) is essentially independent of the temperature(since it is a pure electronic tunneling).

Figure 5. Representative testbeds used to measure the electrical proper-tiesof SAMs. A) Scanningtunneling microscopy. Reprinted withpermissionfrom[155]. Copyright2004, American ChemicalSociety. B) Hg-SAM-SAM-Metal(Au) device. C) Crossed wires (Au wires). Figure 1b and 1c reproduced withpermission from [120]. Copyright 2008, Institute of Physics. D) Metal-insulator-semiconductor. Reprinted with permission from [174]. Copyright2006, American Physical Society. E) Nanopore. F) Nanoparticle-SAM-metal(Au) device structure. Figure 1e and 1f reprinted with permission from [155].Copyright 2004, American Chemical Society.



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As an example, conduction through SAMs derived fromv-substituted aromatic or heteroaromatic alkanethiols (Fig. 6A)was investigated in AuSAMAu junctions, where the top Auelectrode was deposited by cathodic deposition directly on top ofthe monolayer.[176] High breakdown field strengths (EB)>50MVcm1 were measured, and it was concluded that the terminalaromatic rings have strong pp interactions which densify the

surface, hindering the penetration of metal. Furthermore, theauthors modified the typical square-well tunneling potential to fitthe JV data within the direct tunneling model, but did notinvestigate the J(V, T) dependence. In a recent study, edgemolecular junctions were used to investigate the electricalproperties of conjugated 4,40-biphenyldithiol (BPDT) SAMscompared to an aliphatic C9 dithiol alkyl SAMs (Fig. 6B).

[177]

The top electrode (either Ag or Au) was vapor-deposited atreduced temperatures and at a 608 angle to create the edgestructure. Conduction through the conjugated BPDT SAM wasdetermined to be via tunneling, despite the larger currents andsmaller HOMOLUMO gap than in the C9 SAM.

Conduction in SAMs also depends on thenature of the chemical linker that binds themolecule to the bottom metal electrode.[55,178]

For example, the conductance differenceswithin a series of 1,4-butylene alkanes termi-nated with dimethyl phosphine, methyl sulfide,

or amine (Fig. 6C) was compared.

[179]

Theauthors assumed nonresonant tunneling as thedominant conduction mechanism, and foundthat phosphine termination provides the lowestcontact resistance (highest conductance) of theseries. Similarly, a correlation between mole-cular structure and electrical resistance (R) inthe nonresonant tunneling regime was demon-strated in another study by a systematiccomparison of alkyl versus conjugated andmono- versus di-thiol substituted bridgingmolecules (Fig. 6D).[180] Based on a multi-barrier tunneling model,[181] for a given chainlength R, the resistance of the monothiol

junction was roughly 2 greater than thedithiol junction, most likely due to the proper-ties of the chemisorbed versus physisorbednature of the top contact. Interestingly, thecontact resistance of the alkane dithiols andtheir conjugated analogs were found[180] to besimilar and independent of the differingHOMOLUMO gaps of the different molecularstructures.

The conduction mechanism of SAMs onoxide surfaces is far less developed than forSAMs on Au. For example, in 1996 Vuillaumeet al. reported a suppression of tunneling in

metal/alkylsilane/SiO2/Si structures, resultingin low conductivities (which is advantageousfor TFT gate dielectric applications as will bediscussed in Section 4). The conduction wasshown to be independent of monolayer thick-ness (Fig. 7A), indicating that tunneling is notthe dominant conduction mechanism, how-

ever no alternative mechanism was suggested.[182] In 1999,Waldeck and co-workers performed photocurrent measurementson Si/SiO2 coated with different alkylsilanes and observed a muchweaker dependence of the conduction on monolayer thicknessthan expected (Fig. 7B), and the authors suggested hoppingthrough traps in the film (see below) as a possible mechanism toexplain the weak distance dependence.[183] In 2002, Cahen and

co-workers measured the conductance through alkanes in Hg/alkylsilane/SiO2/Si MIS devices.

[165] The results are striking sinceat first glance it would appear that there is a clear dependence ofthe current density on the chain length (Fig. 7C). However, theauthors explained that the curves are essentially identical withinthe experimental uncertainty. The results are then in agreementwith the results from the earlier studies, where the conduction inalkylsilanes on native SiO2 does not depend on chain length.Furthermore, theJvalues of 106 A cm2 at 1 V are lower than thetheoretical model for tunneling, even with an added tunnelingbarrier (where the added barrier was included to account for thepossibility that not all of the silane chains are bonded to the SiO2).

Figure 6. Examples of molecular structures used in Au-SAM-Au measurements.A) v-Substituted aromatic or heteroaromatic alkanethiol used in [176]. B) Illustration of anedgeAu/SAM/Ag device, and corresponding IVcurves for AuC9Ag and AuBPDTAg (ex),and theoretical estimation of the C9 response (Th). Adpated with permission from [177].Copyright 2007, American Institute of Physics. C) Molecular structures of n-butylene diamine,diphosphine, and disulfide molecules used in breakjunctions of [179]. D) Structures of alkylversus conjugated polymeric structures used in [180].



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elucidate the nature of hopping in conjugated SAMs[206] byinvestigating phenylene diisocyanide in similar nanopore devices(Fig. 9B). It was observed that the dominant conductionmechanism depends on the defect level introduced during thefabrication process. For very large defect densities (typically indevices with evaporated metal contacts), only hopping wasobserved with a barrier of $0.3 eV. However both hopping andthermionic emission (see below) were observed when the defectlevel was reduced (with corresponding barrier heights similar tothe previous study of thioacetylbiphenyl SAMs).

The energy distribution of electrons in metals is given by theFermiDirac distribution function. This implies that at elevated

temperatures (and at larger applied biases than for hopping) alarger fraction of the electrons will have sufficient energy tosurmount the energetic barrier presented by the SAM.Thermionic (Schottky) emission (Eq. 8) assumes that an electronfrom the contact can be injected into the dielectric once it hasacquired sufficient thermal energy to cross the potentialmaximum resulting from the superposition of the external andthe imagecharge potential.[186,207] However, if the SAM hasstructural imperfections (such as defects or impurity ions), thesedefect states can act as electron traps. Thermally excited trappedelectrons will contribute to the current density according toPooleFrenkel emission (Eq. 9) at high temperatures and

intermediate fields.[70,172]

JS AT2 expq fs

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqE=4p""0

p kT

0@

1A (8)

JPF / Eexpq fPF

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqE=4p""0

p kT

0@

1A (9)

Since both mechanisms result from Coulombic lowering of thepotential barrier under an applied electric field, the two processeshave similar J(V,T) dependencies. The square root term forPooleFrenkel emission is greater than Schottky emission by afactor of 2, and often the exponent of Eq. 8 and 9 are simpli-fied by introducing a b parameter [bS (q3/4pee)1/2 and bPF (q3/pee)1/2].[208] Here, A* is the modified Richardsons constant(A*

120 A cm2 K2),[186] Eis the electric field, fS and fPF are the

Shottky and PooleFrenkel barrier heights, e is the dielectricpermittivity, e the permittivity of vacuum, and k the Boltzmannconstant.[152]

As an example, thermally activated defect conduction was alsoobserved in cross-linkable organo-siloxane hybrid dielectrics,synthesized by a solgel process.[209] The leakage currentcharacteristics were described by a PooleFrenkel emissionmechanism, related to traps in the bulk of the film. In this case,the films are much thicker (260 nm) and the material is notself-assembled. However, since the Cap layer (Fig. 4) of SANDs isalso a crosslinked siloxane, it is worthwhile to mention briefly theelectronic properties of these alkoxysilane hybrid dielectrics.From ATR-FTIR spectra (Fig. 10A), the authors observed a

reduction of the silanol n(SiOH) peak intensity and enhance-ment of the transition corresponding to the siloxane bondn(SiOSi) with increasing film annealing temperatures (150,170, and 190 8C). Furthermore, the leakage current density(106105 A cm2 at 2 V) is lower for the films annealed at190 8C versus those annealed at 150 8C. The authors fit the JVdata according to the PooleFrenkel mechanism and found thatthe b parameter value extracted from the slope of the fits matchedthe b parameter estimated from bPF (q3/pee)1/2. SincePooleFrenkel emission is related to thermal excitation oftrapped electrons into the insulator conduction band, and sinceit has been reported that electrons can be deeply trapped inhydrated thermal silicon oxides (forming SiO),[210] the authorsproposed that the silanol groups act as trap sites (Fig. 10B) in the

siloxane-based dielectric. Recently, inkjet printing of thesematerials for gate dielectrics in TFTs was demonstrated,[211]

which is important for the low cost development of low leakagecurrent gate dielectrics. Deposition of a high capacitanceself-assembled SAM has yet to be demonstrated, although veryrecently inkjet printed source/drain electrodes were utilized intop contact TFTs with a SAM-based gate dielectric,[212] andorganic single-crystal TFT arrays were demonstrated by reliefprinting of thick ($13 nm) OTS films.[213]

Recently, the conduction mechanism of 3-aminopropyltri-methoxysilane (APTMS) SAMTs on Si/SiO2 was investigated, andit was found that as thickness of the SAMTs is increased, the

Figure 9. Molecular structures of SAM precursors and correspondingJ(V,T) characteristics in Arrhenius plots for A) thioacetylbiphenyl. Rep-rinted with permission from [205]. Copyright 1997, American Institute ofPhysics. B) Phenylene diisocyanide. Reprinted with permission from [206].Copyright 1996, Elsevier.



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current densities do not follow the typical dependence fortunneling (Fig. 10B).[214] Also, hysteresis and negative differentialresistance[215,216] were observed from the SAMT devices, but notfrom the monolayer devices. The authors suggested that thetransport in APTMS SAMTs is hopping. The Northwestern grouphas also investigated annealing conditions and conduction

mechanisms in SAMTs (Fig. 4). For example, it was found thatSAND III leakage current densities do not change significantlywith increasing annealing temperatures (seeSection 4.3.1.1 for results with multiple layersof III),[217] suggesting similar transport inSAND Cap layers to the crosslinked alkoxysi-lane network described above. To further probethe conduction of SANDs, the J(V,T) transportcharacteristics of for II and III were studiedover the temperature range of65100 8C. Wefind (in unpublished data) slightly differentconduction in II versus III as might be expectedfrom the different interfaces (p vs. s, respec-tively) and thicknesses (3.2 and

$6.0 nm,

respectively). Analysis of the J(V,T) character-istics according to tunneling and injectionmodels will be reported shortly.

4. TFT Device Applications

In the previous sections we introduced theimportance of molecular orbital alignment,since SAMs with different HOMOLUMOgaps and/or dipoles can shift the metal workfunction/semiconductor electron affinity and

change the barrier to conduction.[54,218,219] Inthis section, we highlight some experimentalresults where SAMs are used as surfacetreatments in devices to modify electroninjection.[220]

4.1. Metal Surfaces/Bottom Contact TFTs

Practical device technologies will most likelydepend on prepatterning of the metal electro-des for bottom contact TFTs and integratedcircuits. In this configuration, the majorchallenge is increased contact resistance arisingfrom several effects such as interfacial chargemigration, surface dipoles, the insulating natureof semiconductor side chains, or physicaldelamination/dewetting.[221] Therefore, a majorpotential enhancement of SAMs in bottomcontact TFTs is the possibility of reducing

contact resistances by enhancing the semicon-ductor adhesion and growth orientation relativeto the metal source/drain electrodes.[64,222] Forthe commonly used organic semiconductor,pentacene (P5, Fig. 1), surface modification isessential to enhancing bottom contact TFTperformance since optimal wetting of the

substrate is essential for favorable large-grained first layer P5growth.[221,223]

As an example, surface treatment of Au bottom contact source/drain electrodes with simple alkanethiols (such as1-hexadecanethiol) have been shown to modify the surfaceenergy such that large P5 crystal grains form over large areasboth on top of and between the source/drain electrodes. TheseTFTs exhibit a significant increase in P5 mobility (Fig. 11A) from0.16 cm2 V1 s1 (without a SAM) to 0.48 cm2 V1 s1 (with a

Figure 11. A) Pentacene TFT output characteristics for untreated (top) and SAM treated(bottom) Au bottom contact source/drain electrodes. Reprinted with permission from [223].Copyright 2001, IEEE. B) AFM images of pentacene grown on an untreated (top), aliphatic SAMtreated (middle), and aromatic SAM treated (bottom) bottom-contact Au source/drain electrodes.Reprinted with permission from [225]. Copyright 2006, American Institute of Physics.

Figure 10. A) ATR-FTIR spectra of the solgel alkylsiloxane dielectrics, and cartoon of theproposed SiO electron traps. Adapted with permission from [209]. Copyright 2006, AmericanInstitute of Physics. B) Semilog plot of measuredJ as a function of the multilayer thickness (squares),and calculated theoretical J assuming tunneling (circles), and illustration of the APTMS multilayerstructure. Reprinted with permission from [214]. Copyright 2008, Springer.



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SAM).[223] However, alkanethiols in general have a large energygap between the HOMO and the LUMO, typically Eg $ 6 eV,[224]and hence are good insulators.[225] Due to the insulating behavior,reduced charge carrier injection is expected, which makesalkanethiols not suitable as contact primers for OTFTs. Incontrast, aromatic SAMs have smaller MO gaps and hence are of

interest for functionalization of bottom contact electrodes inTFTs.To this end, several groups focused on comparing the effects of

aliphatic versus aromatic SAMs on the device performance ofbottom contact TFTs (see Fig. 2, e.g., of molecular structures). Thetwo major advantages of using aromatic SAMs instead of aliphaticSAMs are increased Ion:Ioff ratios due to favorable electroninjection,[226228] and increased wettability of the SAM surface bythe semiconductor due to SAM-semiconductor attractive inter-actions (Fig. 11B),[225] both of which lower the contact resistanceand improve TFT performance.[229,230] In this example, generallyimproved bottom contact TFT device characteristics wereachieved with an aromatic anthracene-2-thiol SAM (on/off ratioof 106 and strongly reduced trap density as indicated by the low

subthreshold swing of 0.55 V decade1). AFM and SEM imagesrevealed significant dewetting of P5 films on untreated Auelectrodes, but no dewetting on both aliphatic and aromatic thiolSAM treated Au electrodes (Fig. 11B). The authors concluded thatthe contact resistance observed in TFTs without SAMs is relatedto morphological irregularities. However, since the P5 filmmorphology is similar on both aromatic andaliphatic SAMs, the contact resistance in TFTswith aliphatic SAMs must arise from poorelectron injection.[225]

In an effort to correlate molecular structurewith TFT performance, Katz et al. screened aseries of thiols and other commercially avail-

able sulfur reagents with a variety of terminalfunctional groups as surface treatments forTFTs using a fluorinated naphthalenediimidederivative as the semiconductor layer.[221]

Several of the thiol treatments provided abeneficial effect compared to devices withuntreated electrodes. The highest TFT ISDcurrents were obtained with 2-chlorobenzylmercaptan, and the current enhancementswere attributed to increased wetting or stickingof the semiconductor to the thiol-treated goldsurface versus bare gold, due either to favorablearylaryl interactions or to COOH-carbonylhydrogen bonding between the SAMs and the

semiconductor (rather than due to differentdipole moment strengths).

In a different approach for SAM-Au treat-ment, lower contact resistance was demon-strated by directly coupling a molecularmonolayer similar in structure to that of P5(the thioketone in Fig. 12A) to the source/drainelectrode surface prior to P5 deposition.[229] Inthis configuration, the semiconductor SAMsurface treatment acts as a template forenhanced P5 growth, and significant enhance-ment of TFT response characteristics over the

untreated and small molecule-thiol treated bottom contact TFTswas observed. Recently, injection barrier effects of various SAMs[thiophenol (TP), 4-fluorothiophenol (4-FTP), or pentafluorothio-phenol (PFTP)] on bottom contact TFT performance wereinvestigated using TIPS-pentacene as the semiconductor (seeFig. 12B for molecular structures, TFT electrical characteristics,

and proposed energy level diagram).

[227]

No TIPS-pentacene filmmorphology change was observed for films deposited on both theSAM-treated and bare Ag electrodes. However, a significant workfunction shift was measured (from 4.14 to 5.35 eV) by electrodetreatment with the SAMs, and as a result enhanced TFTperformance with SAM treated electrodes was attributed to betteralignment of the modified metal work function with thesemiconductor HOMO level. In untreated devices the Ag workfunction (4.7 eV) and semiconductor HOMO energy level (5.3 eV)mismatch creates a hole injection barrier of 0.6 eV. However, Agsurface modification with 4-FTP or PFTP dipolar SAMs modifiesthe work function to 5.21 and 5.35 eV, respectively, significantlyreducing the injection barrier and creating an ohmic contact withreduced contact resistance.

4.2. Oxide Surfaces/Top Contact TFTs

One overall goal in the organic electronics field has been toenhance OTFT performance to reach metrics comparable to

Figure 12. A) Thioketone used for SAM, and cartoon of electrode/SAM/pentacene interface(top). TFT transfer characteristics (bottom) for devices with SAM (red) and without SAM (blue).Reprinted with permission from.[229] Copyright 2006, American Chemical Society. B) Bottomcontact TIPS-pentacene TFT output with SAMs of TP (top) and PFTP (middle) on the bottom Agsource/drain electrodes, and corresponding energy level diagram (bottom). Reprinted withpermission from[228]. Copyright 2008, American Institute of Physics.



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amorphous hydrogenated silicon (a-Si:H), the current materialused in fabricating thin film transistors (TFTs) for LC displays,which exhibits an electron carrier mobility of$1.0cm2 V1 s1and an Ion:Ioff ratio>10

6. P5 is one of the most promising organicsemiconductors for use as the active material in OTFTs because ofits relatively high mobility and large TFT Ion:Ioff ratios, which arecomparable to or surpass those of amorphous silicon.[231] Thus,many studies have employed SAM-modified SiO2 as the gatedielectric and P5 as the semiconductor (Table 1) to elucidatefundamental correlations between dielectricsemiconductorinterfacial effects and TFT performance (see ref. [42] for a recent

review).

[232]

Typically it is observed that SAMs (as gate dielectricsurface treatments) in OTFTs achieve one or more of thefollowing: reduce the device subthreshold slope, increase thesemiconductor mobility, increase the Ion: Ioff ratio, alter thepolarity of the majority charge carrier type, modulate the carrierdensity in the channel, and shift the threshold voltage (comparedto control devices with bare gate dielectric oxides).[65,232]

Explanations for the overall performance enhancements areusually described in the context of the semiconductor film growthmorphology[35,231] (crystallinity and grain size), and/or thedielectricsemiconductor interfacial properties[233] (such as sur-face energy, trap sites, molecular chemical functionality, and thestructure of the SAM itself).

The first indications that the dielectricsemiconductor inter-

face has a large influence on the overall OTFT performance werethe observations of: (i) different measured mobility values for thesame organic semiconductor, and (ii) different film growthcharacteristics on different types of gate dielectrics. For example,in 1997 Gundlach and co-workers deposited P5 on LiF and SiO2surfaces, and reported distinct differences in the AFM images ofP5 films of varying thicknesses.[234] In 1992 Horowitz et al.measured different mobility values for films of the 6T(sexithiophene, Fig. 1) semiconductor on different oxide andpolymer gate dielectrics,[235] and in 2005 Muccini and co-workersstudied the supermolecular organization of thin films of 6T onSiO2 for a correlation between intermolecular interactions and

transport properties as compared to 6Tsingle crystals, which havemobilities up to 20 cm2 V1 s1.[236] In a later study Horowitzet al. used AFM images of very thin (1 nm) to thicker (7.5 nm) P5films deposited on top of 1-phosphonooctane SAMs (using Al/Al2O3 substrates) to confirm that the P5 growth mode is differentwhen the growth is near the dielectric surface compared togrowth in the bulk (Fig. 13A).[237] In this case, the average P5mobility was 2 cm2 V1 s1 (largest value $3 cm2 V1 s1 on the1-phosphonohexadecane SAM), which was a significant improve-ment over the bare alumina control TFT of 1.1 cm2 V1 s1. Theauthors claimed near single-crystal qualityP5 growth during the

early stages of deposition, which explains the large observedmobilities of the TFTdevices since the first few monolayers of thesemiconductor film are the active charge transporting region.[237]

In 2006, Mottaghi and Horowitz observed similar differences inP5 growth modes on bare versus eicosanoic acid(CH3(CH2)18COOH) SAM modified Al/Al2O3 gate dielec-trics.[238] The P5 films were modeled as multi-layer dielectrics,and it was found that the bulk P5 mobility increases linearly withthe gate voltage up to about 5 V on the SAM-coated Al/Al2O3 and10 V on bare Al/Al2O3, then saturates at large values of $5cm2V1 s1 and $3 cm2 V1 s1, respectively (Fig. 13B), confirmingthat the semiconductor mobility and growth mode are sensitive tothe dielectricsemiconductor interface.

Since, these pioneering studies, many research efforts have

been devoted to the investigation of the dielectricsemiconductorinterface, and major improvements in the OTFT electricalcharacteristics have been documented.[239248] For example,Forrest and co-workers observed major TFT mobility and Ion:Ioffenhancement when using OTS-treated gate dielectrics in P5TFTs, from 0.06 cm2 V1 s1 and Ion:Ioff$ 105 on bare SiO2, to aslarge as 1.2 cm2 V1 s1 and Ion:Ioff$ 108 on OTS treatedSiO2.

[249] Schwartz and co-workers found dramatic improve-ments (Ion:Ioff ratios of 10

8 and subthreshold slopes of 0.2Vdecade1) over other devices using octadecylsilane and otherphosphonates.[250] In this study, a new (anthracene)phosphonateSAM was reported (Fig. 14C), and compared to the other

Table 1. Comparison of pentacene OTFT performance on various oxide gate dielectric surface treatment SAMs (m [cm2 V1 s1], current on/off ratio Ion/Ioff, threshold voltage VT [V], and subthreshold slope SS [V decade

1]).

Reference SAM Oxide m (Ion/Ioff) VT (SS) Year[233] OTS SiO2 0.4 (107) (1.6) 1997[248] OTS iO2 0.6 (107) (0.6) 2002[236] 1-Phosphonooctane

Al2O3 2.1 (1.7

106)

2.1(2.5) 2003

[257] Alkylsilane SiO2 0.13 (107) 5.0 2004Perfluoroalkylsilane SiO2 0.20(108) 17Aminoalkylsilane SiO2 2.4 103(108) 11

[251] HMDS SiO2 0.2 (105) 0.8 2005[242] poly(imide-Piloxane) SiO2 2005

n 0 0.11(104) 8(3.1)n 52 0.25(105) 7.2(2.1)n 130 0.56(5 105) 5.4(1.7)

[237] Eicosanoic acid Al2O3 5 1.2 2006[239] HMDS SiO2 0.5(106) 1.1(2.9) 2006

HMDS Al2O3 0.01(105) 1.2(4.1)[249] (9-Anthracene)phosphonate SiO2 (108) 4.5(0.2) 2007[254] b-Phenethyltrichlorosilane 1.5(106) 18 2007[258] ODTS disordered SiO2 0.3(106) 2008

ODTS ordered

SiO2

0.6(106)



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organophosphonate SAMs tested. The anthracene-based SAMexhibited the best TFT performance. The authors ascribed theimprovements to structural similarities between P5 and the SAMsubstituent, which resembles a continuous anthracene film, thusproviding nucleation sites for favorable growth of P5 islands.Similarly, Mottaghi and Horowitz[238] explained OTFT perfor-

mance enhancements related to P5 grain size using the multipletrapping and release model. If the initial increase in bulk P5mobility with gate voltage can be explained by filling of trapslocated in grain boundaries, then the smaller, more regular P5

grains observed on SAM treated Al/Al2O3should (and do) perform better than the larger,more irregular grains observed on bare Al/Al2O3.

Correlations between P5 film grain size/crystallinity and TFTmobility have been studied

in great detail,

[239,249,251255]

however exacttrends remain unclear,[1,65,256] and the behavioris entangled with other effects (surfaceenergy,[36] surface roughness,[257] and molecularstructure[258]). For example, high aqueouscontact angles (8081158), or more hydrophobicsubstrate surfaces, typically result in the largestOTFT performance increase. However, it is notalways simple to isolate the unambiguous originof the improved performance. Cho andco-workers, deposited P5 on ordered (alkylchains aligned and packed) and disordered(loosely oriented alkyl chains) ODTS SAMs(Fig. 15A), and a barrage of structural techni-

ques were used to analyze the structural andmorphological characteristics of both the ODTSand the P5 films.[259] While the surface energyand surface roughness of the two ODTS SAMswere measured to be the same, the OTFTperformance was significantly different, withenhanced performance observed in the more

ordered SAM. X-ray diffraction spectra (Fig. 15B) of the P5 filmsrevealed more crystalline P5 films when deposited on orderedversus disordered SAMs. Furthermore, a larger diffraction peakintensity ratio of the thin-film phase to the bulk phase wasobserved for the more ordered SAMs, indicating that P5deposited on the more ordered ODTS monolayer is more

sensitive to interactions with the surface since the thin-film phaseis the surface-induced (strained) phase.[259] In another example,the grain size of P5 films was reported to be unchanged whendeposited on HMDS-treated SiO2 and bare SiO2; nevertheless,

differences in the OTFT performance charac-teristics were again observed.[252] In this case, itwas proposed that the suppressed off-currentsobserved for HMDS-treated devices were dueto a reduction in the density of interfacialtrapping states.[260,261]

Strong evidence for this interfacial trappingaffect was presented by Friend and co-workersin 2005.[210] In this study, the authors usedmultiple-reflection attenuated-total-reflection

FTIR spectrometry (ATR-FTIR) to track thechanges of the SiOH stretching and bendingcombination band near 38004 700 cm1 withtime and applied voltage. Based on the longtimes associated with peak intensity changesand wavelength shifts compared to the time forloss of TFT activity, the authors proposed thatthe spectral shifts were due to the generation ofSiO induced by deeply trapped electrons,indicating electrochemical trapping of elec-trons by SiOH surface groups. The interfacialtrapping effect was observed in TFTs using the

Figure 14. OTFT characteristics for pentacene deposited on SiO2/SAM dielectrics of A)n-octadecylphosphonate (ODPA), B) (quarterthiophene)phosphonate, and C) (anthracene)pho-sphonate. Adapted with permission from [250]. Copyright 2007, American Chemical Society.

Figure 13. A) AFM images of pentacene films of thickness 1nm (top), 3.5 nm (middle), and7.5 nm (bottom) grown on 1-phosphonohexadecane SAM. Reprinted with permission from[237]. Copyright 2003, American Chemical Society. B) Cartoon of pentacene growth modedifferences and mobility saturation versus gate voltage for SAM and bare Al2O3 gate dielectrics.Reprinted with permission from [238]. Copyright 2006, Elsevier.



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polymer F8BT [poly(9,90-dioctylfluorene- co-benzothiadiazole)] asthe semiconductor and alkyl-SAMs of various lengths: hexam-ethyldisilazane (C1), decyltrichlorosilane (C10) and ODTS (C18)as Si/SiO2 gate dielectric surface treatments. Initially, enhance-ment of the n-channel TFT performance was observed withincreasing alkyl-SAM chain length (Fig. 16) and surface OHpassivation. However, TFT currents eventually degraded overtime, and since siloxane-terminated SAMs cannot completelyeliminate surface SiOH groups, the degradation was attributed tothose trap sites at or near the SiO2 interface. This study not onlydemonstrates the importance of the dielectricsemiconductorinterface on TFT performance, but also demonstrates that SAMmodification benefits not only p-type (P5) TFTcharacteristics but

also a wide range of n-type semiconductor materials.[7,8,244,262]Lastly, the chemical composition of the semiconductor

dielectric interface can directly tune the semiconductor char-acteristics in a different way. In addition to charge carrierinversion (from p- to n-), it has been shown that the chemicalstructure of the SAM molecules can modify the carrier density inthe semiconductor channel and induce appreciable thresholdvoltage (VT) shifts. For example, P5 and C60 ISD currents wereobserved to depend strongly on the SAM molecular structure,where the currents at VG 0 V were enhanced by six orders ofmagnitude in devices with SAMs composed of perfluoroalk-ylsilane molecules compared to devices fabricated with aminoalk-

ylsilane SAMs (Fig. 17A).[258] The VT values were shifted for theamino- and fluoro-functionalized SAMs, however the VT valuesfor the unsubstituted alkylsilane SAM and untreated devices wereessentially the same, suggesting that the additional carriers in thesemiconductor channel are generated by the built-in potentialscreated by the different SAM dipole moments.

In 2007, Katz and co-workers capitalized on this effect tofabricate complimentary circuits.[263] Using a nonpolar silane,PTS (phenyltrimethoxysilane), and a series of fluorinated dipolarsilanes trichloro(3,3,3-trifluoropropyl)silane (FPTS), trichloro1H,1H,2H,2H-perfluorooctyl)-silane (FOTS), and trichloro(1H,1H,2H,2H-perfluorodecyl)silane (FDTS), they demonstrated controlover the doping level of the p-type semiconductor 5,50-bis(4-hexylphenyl)-2,20-bithiophene (6PTTP6) and fabricated uni-polar inverters (see Fig. 17B for structures). Where theenhancement-mode TFT uses the p-type semiconductor 6PTTP6with the nonpolar PTS SAM (since this SAM induces a slightnegative VT shift the device is normally-off), and thedepletion-mode TFT uses 6PTTP6 with the polar fluorosilanes(since these SAMs exhibit a positive VT shift they are normally

on). The conclusion is that SAMs can induce a controllable VTshift and modulate the current in logic circuits without the needfor external electric field poling. This represents one of the bestexamples of how fundamental insights concerning the SAM-semiconductor interaction enable technological advances.

An alternative surface functionalization approach has recentlybeen demonstrated by Podzorov and co-workers,[264] whereorganosilane SAMs deposited on the surface of single crystalrubrene (Fig. 1) induce a pronounced increase of the surfaceconductivity s 105 S square1, compared to 108107 Ssquare1 typically observed in OTFTs.[258] In this work, top gateOTFTs were fabricated on the SAM-functionalized single crystals

Figure 16. F8BT polymer semiconductor TFTs with various organosilaneSAMs on 200 nm SiO2 as the gate dielectric, or with polyethylene on SiO2as a buffer. Reprinted with permission from [210]. Copyright 2005,Macmillan Publishers Ltd.

Figure 15. A) Cartoon depicting the difference between ordered (left) anddisordered (right) ODTS monolayers. B) X-ray diffraction patterns of the 50nm-thick P5 films deposited on ordered (top) and disordered (bottom)ODTS monolayers at 30, 60, and 90 8C. The inset shows the enlarged thinfilm (002) and bulk (002)0 phase reflections of the XRD pattern. Reprintedwith permission from [259]. Copyright 2008, American Chemical Society.



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using parylene as the gate insulator andobserved ms of 12cm2 V1 s1. Althoughthese mobilities are lower than the highestreported,[265] and the details about SAM growthand surface binding remain unclear, theenhanced conductivity observed with SAM

functionalization is promising for top gateOTFT applications utilizing the SAM as thegate insulator (instead of as a surface treatment)between the single crystal channel beneath andthe deposited gate electrode on top.

4.9. SAMs and SAMTs As Gate Insulators

The notion of high capacitance SAMs and theiruse as the gate dielectric in OTFTs (Table 2) waspioneered by Vuillaume et al. in 1996 where itwas established that SAMs of n-ODTS graftedon Si native oxide were good insulators

(J$ 108 A cm2 at 5.8 MV cm1 and break-downat 912 MVcm1) despite a thicknesses ofonly 2.8 nm.[38] The authors demonstrated thatthe interface state density (D) of the OTS SAMscan be reduced by 1 order of magnitude (to2 1011 cm2 eV1) by annealing the film at350 8C.[266] They also investigated the electricalproperties of alkyl monolayers versus varyingchain lengths (1.92.6nm) and found sup-pressed leakage current densities and lowconductivities ($4.6 1015 S cm1) for well-ordered SAMs, and much larger conductivitiesin deliberately disordered SAMs.[182] One of thefirst attempts utilizing SAMs of alkylsilaneswith a COOH end group as the gate insulatorin a p-type (sexithiophene, 6T) OTFT wasdemonstrated with respectable performancemetrics (m$ 3.6 104 cm2 V1 s1, Ion:Ioffratio $104, and VT 1.3V).[267] However, TFTswith an aromatic terminated alkylsilane,18-phenoxyoctadecyltrichlorosilane (PhO-OTS),

Table 2. Summary of the capacitance C[nF cm2], dielectric constant eeff, and OTFT characteristics (gate voltage VG [V], m [cm2 V1 s1], current on/off

ratio Ion/Ioff, threshold voltage VT [V], and subthreshold slope SS [mV decade1]) for various SAM and SAMT gate dielectrics.

Reference SAM C Semicond VG m (Ion/Ioff) VT (SS)

[265]

SiCl3(CH2)12COOH 6T 2 3.6 104

(104

) 1.3 (350)[39] PhO-OTS 500 P5 2.5 1.0 (106) 1.3 (100)[37] SAND I 400 6T 1 0.04 (8 102) 0.03

SAND II 710 6T 1 0.02 (7 102) 0.08SAND III 390 6T 1 0.06 (103) 0.06

[268] v-SAND 1 400 P5 2 1.9 ($105) 1.0v-SAND 2 400 P5 2 3.4 ($105) 1.0

[271] Phenylundecanoic acid Al2O3 (6 nm) P5 60 0.35 9[273] 7-OTS/Ti SAMT P5 1 1.3 (500) 0.48[285] ODPA Al2O3 (3.6 nm) 700 P5 3 0.6 (107) (100)[272] ODPA HfO2 (3.1 nm) 580 P5 1.5 0.15 (105) 0.53(130)

sp-PA1 HfO2 (3.1 nm) 690 P5 1.5 0.22 (106) 0.41(110)sp-PA2 HfO2 (3.1 nm) 640 P5 1.5 0.15 (106) 0.41(100)

Figure 17. A) Chemical structures of SAM precursor molecules as the SiO2 gate dielectriccoating in OTFTs (top). Pentacene (middle) and C60 (bottom) TFT transfer characteristics.Dielectrics are: bare SiO2 (untreated), CH3 terminated SAM/SiO2, NH2 terminated SAM/SiO2,and fluorinated SAM/SiO2. Adapated with permission from [258]. Copyright 2004, Macmillan

Publishers Ltd. B) Chemical structures of the SAM molecular precursors and the semiconductor(top). Illustration of a unipolar inverter (middle) and gains of the PMOS-like inverter circuit(bottom, f 10 mHz). The arrows indicate the hysteresis direction of the inverter output; thesupply voltage VDD is 50 V. Reprinted with permission from [263]. Copyright 2007, AmericanChemical Society.



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SAM gate dielectric were investigated by a different group, anddrastically enhanced P5 TFT performance was reported.[39]

Operating at 2 V, the devices exhibited m 1 cm2 V1 s1, a veryhigh on/off ratio of$106, VT $ 1.3V, and a subthreshold slope of100 mV decade1. The OTS-OPh SAM is tightly-packed due tointra-SAM pp interactions and exhibits very low leakage currents

($8 107

A cm

2

at 2.5 V), large breakdown fields andcapacitances of$14 MV cm1 and 900 nF cm2, respectively.The Northwestern group investigated SAMTs (i.e., SAND types

I, II, and III described in Section 2.4.1, Fig. 4) as gate dielectricmaterials. SANDs were established as excellent insulators viasolution-phase cyclic voltammetry and MIS leakage currentmeasurements (current densities in the range of 108105 A cm2),where measured breakdown fields for IIII were $57 MV cm1.[37]Capacitancevoltage (CV) measurements on MIS structuresreveal maximum capacitances Ci 400 (I); 710 (II); 390 (III) nFcm2 at 102 Hz. It was found that annealing at 120180 8Creduces CV hysteresis width to 0.1 V and reduces frequency-dependent CV dispersion, suggesting that pristine IIIIcontain quantities of fixed positive charge densities (Qf),

2 10125 1012 cm2.[266,268] Interface state densities (D)calculated from capacitancevoltage (CV) plots were 3 1012eV1 cm2, and it was found that annealing (120180 8C) reducesQf and D to 10

11 cm2 and 1011 eV1 cm2, respectively. Variousoligothiophene semiconductors and n-type CuPc TFTs werefabricated with SANDs as the gate dielectrics, and comparablemobilities were measured to those obtained on 300 nm SiO2, butat much lower operating voltages for the devices with SAND gatedielectrics. It was noted that TFTperformance could be improvedby patterning the gate electrode, or by incorporating higher-kmolecules in the SAND gate dielectric.

To enhance the TFT performance of SAND gated TFTs, a newself-assembly procedure for the fabrication of multilayer SAND-

like gate dielectrics via room temperature vapor phase deposition(v-SAND, Fig. 18) was demonstrated.[269] In addition, thestructures of the new molecular constituents (1 and 2) areimproved over the original molecular component (Stb) of SANDsby (i) their ability to self-assemble via head-to-tail intra-molecularhydrogen-bonds, and (ii) anticipated larger molecular polariz-abilities than Stb. Here, the trends in dielectric permittivities areevaluated qualitatively using the ClausiusMossotti relatione/ (3 2aN)/(3 aN) (where a is the polarizability along theconjugated long axis of the molecule and N is the moleculardensity in cm3) in conjunction with sum-over-states[270]

calculated molecular polarizabilities. Uniform films (rms rough-ness


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Recently, hybrid inorganic/phosphoric acid SAMs were usedas gate dielectrics in low-voltage OTFTs.[273] In this contribution,phosphonic acid (PA) SAMs deposited on SiO2 (1.7 nm)/HfO2(3.1 nm thick) substrates were used as the gate dielectric in P5TFTs. Phosphoric acid SAMs were used because of their betterstability to moisture and less homocondenstation during devicefabrication compared to their silane-based counterparts. TheSAM molecular structures investigated in this study (Fig. 19B)combine the advantages of phosphoric acid SAMs discussedpreviously (such as PhO-OTS, and anthracene-COOH) byincorporating a long insulating carbon chain with a polarizableanthracene terminal group in both (2-anthryl)undecoxycarbonyl-

decylphosphonic acid (ps-PA1) and (2-anthryl)undecoxycarbonyl-undecylphosphonic acid (ps-PA2). The hybrid HfO2/PA-SAMsexhibit large capacitances of 690 nF cm2 (ps-PA1) and 640 nFcm2 (ps-PA2) compared to SAMs ofn-octadecylphosphonic acid(ODPA, 580 nF cm2), and low leakage current densities of109108 A cm2 at 2 V, allowing for a P5 TFT operating voltageof only1.5 V. Improvements in the P5 TFTperformance such aslarger mobilities, enhanced Ion:Ioff ratios, and lower subthresholdslopes ($100 mV decade1) were observed for the ps-PA-basedOTFTs compared to the bare HfO2 control devices, and wereexplained by a combination of surface energy and chemicalfunctionality at the P5/dielectric interface.

In another example of hybrid organi-c-inorganic gate dielectrics, rapid vapor-phasefabrication techniques were used to buildsuperlattices of alkene-terminated SAMs withTiOH interlayers.[274] The hybrid SAMs wereformed by exposing TiO2-coated Si substrates

to 7-octenyltrichlorosilane (7- OTS) in thepresence of H2O vapor at temperatures of100 and 20 8C. The terminal SAM vinyl groupswere converted to carboxylic groups with ozonetreatment in the ALD (atomic layer deposition)growth chamber. SAMTs were then built up bygenerating an active titanium hydroxide layeron the COOH-terminated SAM by vapor phasetitanium isopropoxide adsorption, followed byan exchange reaction with water. SAMT filmswere fabricated by repetition of these threesteps (Fig. 19C). Leakage current density versusvoltage characteristics of the Pt/multilayerSAM/Pt capacitors were measured for SAM

thicknesses from 1.199 nm (Fig. 19C). P5TFTs using the 100 nm-thick multilayer SAMs(further treated with a hydrophobic SAM as asurface treatment), exhibitm 1.3cm2 V1 s1,at operating voltage of only 1 V (due to highdielectric constant of the SAM 17), Ion:Ioffcurrent ratio >500, and VT 0.48 V. Further-more, the authors demonstrated two-terminalelectrical bistable devices, which can beswitched on and off reproducibly more than106 times due to charge filling and defilling ofthe defects in the TiO2 layer or interfaciallayers.

Similarly, the effects of multilayers of SANDtype III (named III-n, where n indicates thenumber of repeating SAND III units, typically n 3, andd$ 16 nm) as the gate dielectric in TFTs have been intenselyinvestigated. SAND robustness studies were begun by firstdemonstrating the compatibility of III-3 with carbon nanotubesemiconductors for low voltage TFTs.[275] Here the SANDmultilayers ($16 nm thick) were deposited conventionally fromthe solution phase, and have a measured capacitance of 170 nFcm2. Single-wall carbon nanotubes (SWCNTs) were grown byCVD onto SiO2/Si wafers and then transfer printed directly ontothe SAND dielectric. The resulting film has $10 tubes mm2which act as the semiconducting film in the TFTs. GoodSWNT-SAND adhesion allows direct photolithographic pattern-

ing of the source and drain electrodes by liftoff. TFTperformanceis greatly improved over control devices using 100 nm SiO2 gatedielectrics as indicated by substantially lowered hysteresis and VTshifts. Since the SiO2 control and SAND have very similar surfaceproperties (such as the number of fixed charges, interface statedensities, and surface chemistry), the enhanced properties of theTFTs were attributed principally to reduced operating voltage,which avoids charge injection traps arising from adsorbed waternear/on the SWCNTs at high voltages. TFT performance isexcellent with m$ 5.6cm2 V1 s1 (in the linear regime),VT 0.2 V, and a low gate leakage current of $10 nA at VG 1 V. In addition, compatibility with n-type SWCNTs (by

Figure 19. A) Molecular structures used in SAMs for gate dielectrics: COOH-anthracene (left)phenylundecanoic acid (right) used in [272]. B) Molecular structures used in SAMs for gatedielectrics: ps-PA1 (right) ps-PA2 (left) used in [273]. C) Schematic of the molecular layerdeposition procedure to build SAMTs (top) and electrical characteristics of SAMTs (bottom). JVplots of different thickness SAMTs in MIM devices (left) and pentacene TFTs (right). Reprintedwith permission from [274]. Copyright 2007, American Chemical Society.



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PEI coating) was demonstrated by the small observed hysteresisand measured TFT properties: m 4.1 cm2 V1 s1 andVT 0.2 V for applications in low voltage complementary logicgates. Significant gains ($8) were measured, which were thehighest at the time compared to 100 nm SiO2 gate dielectrics.

[275]

High-performance single carbon nanotube field effect tran-

sistors with a thin gate dielectric based on a SAM gate dielectric ofPhO-OTS[39] (2 nm thick) were demonstrated by Klauk et al.[276]

This is the first systematic study of the stability of a SAM tovarious e-beam exposures. The authors find negligible change inleakage current density compared to evaporated Au metalcontacts when using an electron dose of 300 mC cm2. However,leakage current increases to >106 A cm2 at doses larger than1800mC cm2. Operating under ambient conditions, the TFTsexhibited large transconductance (20mS), small hysteresis, and alow subthreshold slope (60 mV decade1) from which the authorsestimate a low interface state density (D 6 1011 cm2 V1)compared to other SWCNT transistors. These results demon-strate the important point that organic SAMs can withstandprocessing conditions typically used in the fabrication of

inorganic electronics, which suggests that integration withinorganic semiconductor technologies is anattractive option to overcome challenges ofprinting electronics.

As further examples, the compatibility of theSAND gate dielectrics with an inorganicsemiconductor (In2O3) was demonstrated infully transparent, high-performance TFTs.[277]

In2O3 is a wide-bandgap (3.63.75 eV), n-typesemiconductor (msingle crystal 160cm2 V1s1), with high transparency in the visibleregion (>90%). Thin films of In2O3 (60nm onIII-3, and 120nm on 300 nm SiO2) were

deposited at room temperature by ion-assisteddeposition (IAD) directly on top of the dielectric(note that SAND is stable to the in situ ion/plasma exposure during In2O3 growth). Theconductivities of the thin films were measuredto be 104105 S cm2, and XRD revealedsubstantial crystallinity on both SiO2 andSAND growth surfaces. Significant In2O3TFT performance enhancement is observedwith SAND-gated devices, where m 140 cm2V1 s1, interfacial trap densityD 1011 cm2,VT 0.0 V with nearly hysteretic free response,on/off 105, and the subthreshold slope 150mV decade1, compared to the perfor-

mance on SiO2-gated devices, where m10cm2 V1 s1, on/off 105. To correct forthe performance differences arising from thedifferent dielectric layers, the TFT transfercurrents versus charge carrier density areevaluated. It is found that SAND-based devicesturn on at much lower accumulated chargecarrier densities than SiO2, indicating greatercharge injection efficiency in the former thanthe latter. To realize fully transparent TFTs, thesame fabrication procedures were followedexcept utilizing glass/ITO as the bottom gate

electrode and In2O3 source and drain electrodes. The perfor-mance of SAND-based transparent TFTs is the same as on n-Sisubstrates with an improved subthreshold slope 90mVdecade1. These materials combinations advance the field towardthe realization of plastic electronic displays featuring opticaltransparency, mechanical ruggedness, environmental stability,

and inexpensive/large area fabrication.In further work, it was demonstrated that the powerconsumption efficiency (which is the main technical challengein the development of truly viable flexible displays) of single ZnONW (nanowire)-based TFTs is enhanced by the use of SAND typeIII-3 (16 nm) as the gate dielectric (Fig. 20A).[278] ZnO nanowires(80 nm average diameter, and 5mm average length) werepurchased from Nanolab Inc. The NWs were dispersed in2-propanol and the dispersions were transferred to the SAND-coated Si substrates. Source and drain Al electrodes weredeposited by electron beam evaporation and patterned byphotolithography and lift-off. The SAND dielectrics were firstelectrically characterized in MIS devices (Al/SAND/Si), and aleakage current density

$108 A cm2 and a capacitance of

180 nF cm2 at 1 V were measured, verifying the compatibility

Figure 20. A) Field-emission SEM image of a 130 nm diameter ZnO-NW TFT (the scale bar is2mm) (top), and TFT output characteristics for a SAND-based ZnO-NW TFT (middle), and TFTusing 70nm SiO2 as the gate dielectric (bottom). Reprinted with permission from [278].Copyright 2005, American Chemical Society. B) TFT transfer characteristics for varying protonradiation doses of ZnO-NW TFTs with SiO2 (top) and SAND gate dielectrics (bottom). Reprintedwith permission from [279]. Copyright 2006, American Institute of Physics.



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of SAND with photolithography and e-beam evaporationmethodologies. SAND-gated ZnO2 NW-TFTs showed reducedoperating voltages from 2.5 V to


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as the semiconducting layer.[217] The chemicalbath deposition method (employing Cd2 andsodium selenosulfate, SeSO23 solutions) wasused to deposit the CdSe films on SAND III-3as the gate dielectric. MIS and TFT character-istics were compared as a function of anneal-

ing. It was found that for unannealed, and for300 8C and 400 8C annealed SAND samples,the leakage current density remains low$1.1mA cm2 at 4 V, but the capacitanceincreases with increasing annealing tempera-tures [160 nF cm2 (unannealed)


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authors demonstrated compatibility of the SAM with an n-typeorganic semiconductor hexadecafluorocopperphthalocyanine(F16CuPc), and measured performance metrics similar to thosereported in the literature. Using these SAMs with P5 andF16CuPcorganic semiconductors, low voltage (1.53V) complementarycircuits and ring oscillators were demonstrated with a static

power consumption of less than 1 nW per logic gate. Expandingon this work, Halik and co-workers next demonstrated micro-contact-printed ODPA SAMs as the gate dielectric for TFTs andcomplementary circuits on AlOx/Al substrates (Fig. 23).

[287] Theyfound that TFTs and complementary circuits fabricated byprinting and subsequent wet etching perform identically to theTFTs and ring oscillators reported previously where the gateelectrodes were patterned by shadow-masking. Nonetheless, this isthe first demonstration of a printing process that uses the directprinting of a SAM layer, and subsequently uses that layer as an etchresist to pattern the bottom gate electrodes.

The insight from the above examples of SAM- andSAMT-based TFTs has enabled one of the most unconventionalhybrid organic inorganic electrical technologies reported to date:

transparent active matrix organic light-emitting diode (AMOLED)displays powered by nanowire electronics.[288] This is the firstexample of transparent AMOLED display elements composed of54 176mm OLED pixels, in which the switching and drivingcircuits are comprised exclusively of nanowire transistor (NWT)electronics fabricated at room temperature. Proof of conceptgreen-emitting polymer LEDs with interfacial charge-blockingmaterials were integrated with a transparent bottom contactelectrode. The circuit for a unit pixel consists of one switchingNWT, two parallel driving NWTs, and one storage capacitor. Thedevice is composed of multiple layers making up the transistorand the OLED (Fig. 24). Briefly, the fabrication begins with200 nm thick SiO2 layer deposited by e-beam evaporation on a

glass substrate. Next, 100 nm of ITO is deposited by IAD at roomtemperature, and patterned by photolithography. Next, a 22 nmSAND III multilayer (III-4) is deposited on the patterned ITO gateelectrodes via solution self-assembly. Contact holes are patternedas anode openings for the OLED units and as bottom gate electrodecontacts for the pixel. Next, a suspension of single crystal In2O3nanowires is dispersed on the substrate. ITO source/drain electrodesare deposited by IAD and patternedby lift-off. Next, 200nm of SiO2 isdeposited for PLED fabrication and characterization. The mobilitiesof the present SAND-based In2O3 TFTs (m$ 258 cm2 V1 s1) aresimilar to In2O3 NWs on oxide dielectrics (m$ 6.9279.1 cm2 V1s1) and bulk single crystals (m$ 160cm2 V1 s1). Minor hysteresis,large Ion currents (6 106 A atVG 2.0 V), and low subthresholdslope (0.35 V decade1) indicate negligible charge trapping and

detrapping in/on the SAND and at the NW-SAND interface. Fullytransparent, proof-of-concept 2 mm 2 mm NW-AMOLED arrays(300 pixels 900 NWTs) were fabricated using a very thin Alcathode on glass substrates. The optical transmission values are$72% (before OLED deposition- limited by the thin Al cathode)and $35% (after OLED deposition) in the 3501350nmwavelength range, which corresponds to a green peak lumines-cence of>300 cd m2. Note that transmission coefficients up to70% have been reported for OLED structures on plasticsubstrates.[289] This fabrication process involves fewer masksteps than conventional Si approaches and is very promising forfuture AMOLED displays.

4.4. SAMs as the TFT Semiconductor Channel

As described in Section 4.2, several groups have studied thethickness dependence of mobility for various semiconductors,and for P5 and CuPc the mobilities begin to saturate after 6monolayers of semiconductor deposition.[290,291] This phenom-

Figure 24. A) Illustration of SAND AMOLED with SAND based In2O3-NWTFTs. B) Typical transfer characteristics at different VSD (0.1, 0.2, 0.5, and1.0V), and the inset shows the device hysteresis at VSD 0.1V. C)Measured luminance versus supply voltage curves for a 2 mm 2 mmAMOLED array with all scan and data lines enabled, demonstrating lightemission through the Al cathode (circles) and through the glass substrate(squares). The bottom inset is an optical image of NW-AMOLED consist-ing of three 2 mm 2 mm AMOLED pixel arrays, 340 unit pixels, 80 tran-sistor/circuit test devices, 6 alignment marks, 20 test patterns, and contactpads. Reprinted with permission from [288]. Copyright 2008, AmericanChemical Society.



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enon opens the possibility of using SAMs and SAMTs as theactive channel in TFTs.[292294] In 2005, Pilkuhn and co-workersstudied ODTS SAMs with different end group functionalizations,

such as methyl, thiol, thiophene, phenoxy, and biphenyl.

[292]

Ofparticular interest were the good insulating properties of the alkylchain (large breakdown field EB 16 MV cm1), and thesimultaneous lateral (in-plane) conductance of the I2-dopedbiphenyl end group, suggesting these SAMs could be used asSAM-TFTs. In the same year, Malliaras and co-workers studiedthe dependence ofP5m with film thickness, and found that themobility saturates at$0.45 cm2 V1 s1 after 6 monolayers.[295] In2007, Horowitz and co-workers demonstrated with rather com-plicated device fabrication (e-beam lithography of short channellengths


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Center (DMR-0520513) for support of this research. This article is part of aSpecial Issue on Interfaces in Organic Electronics.

Received: November 6, 2008

[1] A. Facchetti, M.-H. Yoon, T. J. Marks, Adv. Mater. 2005, 17, 1705.[2] T. W. Kelley, P. F. Baude, C. Gerlach, D. E. Ender, D. Muyres, M. A. Haase,

D. E. Vogel, S. D. Theiss, Chem. Mater. 2004, 16, 4413.

[3] S. R. Forrest, Nature 2004, 428, 911.

[4] C. Rolin, K. Vasseur, S. Schols, M. Jouk, G. Duhoux, R. Muller, J. Genoe, P.

Heremans, Appl. Phys. Lett. 2008, 93, 033305.

[5] L. Burgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H.-J. Kirner, C. Winnewisser,

Adv. Mater. 2008, 20, 2217.

[6] A. Babel, Y. Zhu, K.-F. Cheng, W.-C. Chen, S. A. Jenekhe, Adv. Funct. Mater.

2007, 17, 2542.

[7] H. Usta, A. Facchetti, T. J. Marks, J. Am. Chem. Soc. 2008, 130, 8580.

[8] B. Jones, A. Facchetti, M. R. Wasielewski, T. J. Marks, Adv. Funct. Mater.

2008, 18, 1329.

[9] H. Tian, J. Shi, D. Yan, L. Wang, Y. Geng, F. Wang, Adv. Mater. 2006, 18,

2149.

[10] T. Kojima, J.-I. Nishida, S. Tokito, Y. Yamashita, Chem. Lett. 2007, 36,1198.

[11] M. L. Tang, M. E. Roberts, J. J. Locklin, M. M. Ling, Z. Bao, Chem. Mater.

2006, 18, 6250.

[12] R. Schmidt, S. Goettling, D. Leusser, D. Stalke, A.-M. Krause, F. Wurthner,

J. Mater. Chem. 2006, 16, 3708.

[13] M. Mas-Torrent, C. Rovira, Chem. Soc. Rev. 2008, 37, 827.

[14] H. Yan, Y. Zheng, R. Blache, C. Newman, S. Lu, J. Woerle, A. Facchetti,

Adv. Mater. 2008, 9999, 1.

[15] Z. Wang, C. Kim, A. Facchetti, T. J. Marks, J. Am. Chem. Soc. 2007, 129,

13362.

[16] S. E. Koh, J. E. Mendvedeva, A. Facchetti, B. Delley, A. J. Freeman, T. J.

Marks, M. A. Ratner, J. Phys. Chem. B. 2006, 110, 24361.

[17] B. Yoo, D. Basu, T. Jung, D. Fine, B. A. Jones, A. Facchetti, M. R.

Wasielewski, T. J. Marks, K. Dimmier, A. Dodabalapur, Adv. Mater.

2007, 19, 4028.[18] J. A. Letizia, M. R. Salata, C. M. Tribout, A. Facchetti, M. A. Ratner, T. J.

Marks, J. Am. Chem. Soc. 2008, 130, 9679.

[19] H. Kong, D. H. Lee, I.-N. Kang, E. Lim, Y. K. Jung, J.-H. Park, T. Ahn, M. H.

yi, C. E. Park, H.-K. Shim, J. Mater. Chem. 2008, 2008. 1895.

[20] a) Lu, H. Usta, C. Risko, L. Wang, A. Facchetti, M. A. Ratner, T. J. Marks,

J. Am. Chem. Soc. 2008, 130, 7670. b) H. Yan, Z. Chen, Y. Zheng, C. E.

Newman, J. Quin, F. Dolz, M. Kastler, A. Facchetti, Nature 2009, 457, 679.

[21] Organic Semiconductors in Sensor Applications, Springer Series in Materials

Science, Vol. 107, (Eds. D. A. Bernards, R. M. Owens, G. G. Malliaras),

Springer, Berlin/Heidelberg 2008.

[22] J. Weidner, in Proc. ECS Trans, Curran Assoc. Inc., Washington DC 2008,

Vol. 11, pp. 89.

[23] S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew. Chem. Int.

Ed. 2008, 47, 4070.

[24] Organic Electronic Materials: Conjugated Polymers and Low MolecularWeight Organic Solids, (Eds. R. Farchioni, G. Grosso), Springer, Berlin

2001.

[25] G. Malliaras, Organic Semiconductors and Devices, Wiley, Hoboken

2003.

[26] M. Berggren, D. Nilsson, N. D. Robinson, Nat. Mater. 2007, 6, 3.

[27] A. Facchetti, Mater. Today 2007, 10, 28.

[28] A. Facchetti, T. J. Marks, H. E. Katz, J. Veinot, in Printed Organic and

Molecular Electronics, (Eds.: D. R. Gamota, P. Brazis, K. Kalyanasundaram,

J. Zhang), Kluwer Academic, Boston 2004, pp. 83159.

[29] C. R. Newman, C. D. Frisbie, D. A. d.S. Filho, J.-L. Bredas, P. C. Ewbank,K.

R. Mann, Chem. Mater. 2004, 16, 4436.

[30] Printed Organic and Molecular Electronics (Eds.: D. R, Gamota, P. Brazis, K.

Kalyanasundaram, J. Zhang), Kluwer Academic, Boston 2004.

[31] J. Hicks, D. Bergstrom, M. Hattendorf, J. Jopling, J. Maiz, S. Pae, C.

Prasad, J. Wiedemer, Intel Technol. J. 2008, 12, 131.

[32] F.-C. Chen, C.-H. Liao, Appl. Phys. Lett. 2008, 93, 103310.

[33] J. X. Tang, C. S. Lee, M. Y. Chan, S. T. Lee, Appl. Surf. Sci. 2008, 254, 7688.

[34] K. D. Kim, D. S. Kim, C. K. Kim, C. K. Song, Jpn. J. Appl. Phys. 2008, 47,

6496.[35] C. Kim, A. Facchetti, T. J. Marks, Adv. Mater. 2007, 19, 2561.

[36] C. D. Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99.

[37] M.-H. Yoon, A. Facchetti, T. J. Marks, Proc. Natl. Acad. Sci. 2005, 102, 4678.

[38] D. Vuillaume, C. Boulas, J. Collet, J. V. Davidovits, F. Rondelez, Appl. Phys.

Lett. 1996, 69, 1646.

[39] M. Halik, H. Klauk, U. Zschieschang, G. Schmid, C. Dehm, M. Schutz, S.

Maisch, F. Effenberger, M. Brunnbauser, F. Stellacci, Nature 2004, 431, 963.

[40] B. Mann, H. Kuhn, J. Appl. Phys. 1971, 42, 4398.

[41] I. Langmuir, Trans. Faraday Soc. 1920, 15, 62.

[42] M. Kitamura, Y. Arakawa, J. Phys.: Condens. Matter 2008, 20, 184011.

[43] C.-A. Di, G. Yu, Y. Liu, D. Zhu, J. Phys. Chem. B 2007, 111, 14083.

[44] J. Zaumseil, H. Sirringhaus, Chem. Rev. 2007, 107, 1296.

[45] G. Horowitz, J. Mater. Res. 2004, 19, 1946.

[46] J. C. Cho, J. L. Lee, Y. He, B.-S. Kim, T. Lodge, C. D. Frisbie, Adv. Mater.

2008, 20, 686.

[47] Dielectric Films for Advanced Microelectronics (Eds: M. Baklanov, M. Green,

K. Maex ), John Wiley & Sons Ltd., New York 2007.

[48] J. Zhao, K. Uosaki, Appl. Phys. Lett. 2003, 83, 2034.

[49] M. Porti, M. Nafria, X. Aymerich, A. Olbrich, B. Ebersberger, J. Appl. Phys.

2002, 91, 2071.

[50] O. Gershewitz, M. Grinstein, C. N. Sukenik, K. Regev, J. Ghabboun, D.

Cahen, J. Phys. Chem. B 2004, 108, 664.

[51] S. Kera, Y. Yabuuchi, H. Yamane, H. Setoyama, K. K. Okudaira, A. Kahn,

N. Ueno, Phys. Rev. B 2004, 70, 085304.

[52] Y.-J. Lui, H.-Z. Yu, Chem. Phys. Chem. 2003, 4, 335.

[53] A. Scott, D. B. Janes, C. Risko, M. A. Ratner, Appl. Phys. Lett. 2007, 91,

033508.

[54] S. Lenfant, C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, Nano Lett.

2003, 3, 741.

[55] B. Kim, J. M. Beebe, Y. Jun, X.-Y. Zhu, C. D. Frisbie, J. Am. Chem. Soc.

2006, 128, 4970.

[56] K. Demirkan, A. Matthew, C. Weiland, Y. Yao, A. M. Rawlett, J. M. Tour, R.

L. Opila, J. Chem. Phys. 2008, 128, 074705.

[57] S. Lenfant, D. Guerin, F. Tran Van, C. Chevrot, S. Palacin, J. P. Bourgoin,

O. Bouloussa, F. Rondelez, D. Vuillaume, J. Phys. Chem. B. 2006, 110,

13947.

[58] C. Miramond, D. Vuillaume, J. Appl. Phys. 2004, 96, 1529.

[59] N. Peor, R. Sfez, S. Yitzchaik, J. Am. Chem. Soc. 2008, 130, 4158.

[60] H. Vazquez, W. Gao, F. Flores, A. Kahn, Phys. Rev. B Rapid Commun. 2005,

71, 041306.[61] A. Natan, L. Kronik, H. Haick, R. Tung, Adv. Mater. 2007, 19, 4103.

[62] G. Heimel, L. Romaner, E. Zojer, J.-L. Bredas, Nano Lett. 2007, 7, 932.

[63] D. M. Alloway, M. Hofmann, D. L. Smith, N. E. Gruhn, A. L. Graham, R. J.

Colorado, V. H. Wysocki, R. T. Lee, P. A. Lee, N. R. Armstrong, J. Phys.

Chem. B 2003, 107, 11690.

[64] G. Dholakla, M. Meyyappan, A. Facchetti, T. J. Marks, Nano Lett. 2006, 6,

2447.[65] J. Veres, S. Ogier, G. Lloyd, Chem. Mater. 2004, 16, 4543.

[66] G. E. Poirier, E. D. Pylant, Science 1996, 272, 1145.

[67] A. Cassaro, R. Mazzarello, R. Rousseau, L. Casalis, A. Verdini, A. Kohl-

meyer, L. Floreano, S. Scandolo, A. Morgante, M. L. Klein, G. Scoles,

Science 2008, 321, 943.

[68] A. Ulman, Chem. Rev. 1996, 96, 1533.

[69] J. Sagiv, J. Am. Chem. Soc. 1980, 10, 92.

[70] D. K. Aswal, S. Lenfant, D. Guerin, J. V. Yakhmi, D. Vuillaume, Anal. Chim.

Acta 2006, 568, 84.



24/27

www.advmat.de

[71] A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-

Blodgett to Self-Assembly, Academic Press, San Diego 1991.

[72] F. Schreiber, J. Phys.: Condens. Matter 2004, 16, R881.[73] C. Oncins, C. Vericat, F. Sanz, J. Chem. Phys. 2008, 128, 044701.

[74] T. Y. B. Leung, M. C. Gerstenberg, D. J. Lavrich, G. Scoles, F. Schreiber, G.

E. Poirier, Langmuir2000, 16, 549.

[75] C. Vericat, M. E. Vela, G. A. Benitez, J. A. M. Gago, X. Torrelles, R. C.

Salvarezza, J. Phys.: Condens. Matter 2006, 18, R867.[76] S.-S. Li, L.-P. Xu, L.-J. Wan, S.-T. Wang, L. Jiang, J. Phys. Chem. B 2006, 110,

1794.

[77] J. Noh, H. Kato, M. Kawai, M. Hara, J. Phys. Chem. B 2006, 110,

2793.

[78] D. S. Everhart, in Handbook of Applied Surface and Colloid Chemistry, (Ed.

K. Holmberg) Vol. 2 Wiley, Chichester 2003, pp. 22116.

[79] M. Twardowski, R. G. Nuzzo, Langmuir 2002, 18, 5529.

[80] D. Syomin, J. Wang, B. E. Koel, Surf. Sci. 2001, 495, L827.

[81] P. C. Rusu, G. Giovannetti, G. Brocks, J. Phys. Chem. C2007, 111, 14448.

[82] P. E. Laibinis, G. M. Whitesides, D. L. Allara, Y. T. Tao, A. N. Parikh, R. G.

Nuzzo, J. Am. Chem. Soc. 1991, 113, 7152.

[83] A. Shaporenko, A. Ulman, M. Zharnikov, J. Phys. Chem. B 2005, 109,

3898.

[84] E. A. Weiss, R. C. Chiechi, G. K. Kaufman, J. K. Kriebel, Z. Li, M. Duati, M.

A. Rampi, G. M. Whitesides, J. Am. Chem. Soc. 2007, 129, 4336.[85] E. Hoque, J. A. DeRose, R. Houriet, P. Hoffmann, H. J. Mathieu, Chem.

Mater. 2007, 19, 798.

[86] G. K. Jennings, T.-H. Yong, J. C. Munro, P. E. Laibinis, J. Am. Chem. Soc.

2003, 125, 2950.

[87] S. Vollmer, G. Witte, C. Woell, Langmuir 2001, 17, 7560.

[88] J. J. Stapleton, T. A. Daniel, S. Uppili, O. M. Cabarcos, J. Naciri, R.

Shashidhar, D. L. Allara, Langmuir 2005, 21, 11061.

[89] J. C. Love, D. B. Wolfe, R. Haasch, M. L. Chabinyc, K. E. Paul, G. M.

Whitesides, R. G. Nuzzo, J. Am. Chem. Soc. 2003, 125, 2597.

[90] H. Noguchi, M. Ito, K. Uosaki, Chem. Lett. 2005, 34, 950.

[91] S. Lee, J. Park, R. Ragan, S. H. Kim, Z. Lee, D. K. Lim, D. A. A. Ohlberg, R.

S. Williams, J. Am. Chem. Soc. 2006, 128, 5745.

[92] J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides, Chem.

Rev. 2005, 105, 1103.

[93] F. Schreiber, A. Eberhardt, T. Y. B. Leung, P. Schwartz, S. M. Wetterer, D. J.

Lavrich, L. Berman, P. Fenter, P. Eisen Berger, G. Scoles, Phys. Rev. B:

Condens. Matter Mater. Phys. 1998, 57, 12476.

[94] C. D. Bain, E. B. Troughton, Y. T. Tao, J. Evall, G. M. Whitesides, R. G.

Nuzzo, J. Am. Chem. Soc. 1989, 111, 321.

[95] Y. Han, K. Uosaki, Electrochim. Acta 2008, 53, 6196.

[96] J. Zhang, Q. Chi, J. Ulstrup, Langmuir 2006, 22, 6203.

[97] V. B. Engelkes, J. M. Beebe, C. D. Frisbie, J. Am. Chem. Soc. 2004, 126,

14287.

[98] X. D. Cui, A. Primak, X. Zarate, J. Tomfohr, O. F. Sankey, A. L. Moore, T. A.

Moore, D. Gust, L. A. Nagahara, S.M. Lindsay, J. Phys. Chem. B 2002, 106,

8609.

[99] B. Xu, N. J. Tao, Science 2003, 301, 1221.

[100] X. Xiao, B. Xu, N. J. Tao, Nano Lett. 2004, 4, 267.

[101] M. Tsutsui, Y. Teramae, S. Kurokawa, A. Sakai, Appl. Phys. Lett. 2006, 89,

163111.

[102] R. B. Pontes, F. D. Novaes, A. Fazzio, A. J. R. Da Silva, J. Am. Chem. Soc.

2006, 128, 8996.

[103] D. J. Wold, R. Haag, M. A. Rampi,C. D. Frisbie, J.Phys.Chem. B 2002, 106,

2813.

[104] J. G. Kushmerick, D. B. Holt, S. K. Pollack, M. A. Ratner, J. C. Yang, T. L.

Schull, J. Naciri, M. H. Moore, R. Shashidar, J. Am. Chem. Soc. 2002, 124,

10654.

[105] Z.-F. Shi,

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