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Functional Organophosphonate Interfaces for Nanotechnology: A Review Anna Cattani-Scholz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04382 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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Functional Organophosphonate Interfaces for Nanotechnology: A Review Anna Cattani-Scholz1 1

Walter Schottky Institut and Technische Universität München, Germany

KEYWORDS. Organophosphonate Chemistry, Surface Functionalization, Semiconductor Interfaces, Nanotechnology.

ABSTRACT. Optimization of interfaces in inorganic-organic device systems depends strongly on understanding both the molecular processes that are involved in surface modification, and of the effects that such modification have on the electronic states of the material. In particular, the last several years have seen passivation and functionalization of semiconductor surfaces to be strategies to realize devices with superior function by controlling Fermi level energies, band gap magnitudes, and work functions of semiconducting substrates. Among all the synthetic routes and deposition methods available for optimization of functional interfaces in hybrid systems, organophosphonate chemistry has been found to be a powerful tool to control at molecular level the properties of materials in many different applications. In this review, we focus on the relevance of organophosphonate chemistry in nanotechnology, giving an overview about some recent advances in surface modification, interface-engineering, nanostructure optimization, and biointegration.


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1. INTRODUCTION To fulfill the promise of organic electronic devices, performance-limiting factors such as energy discontinuity at materials interfaces must be overcome. Indeed, advances in microelectronics, energy conversion, and sensor design now largely depend on selective modification of device active surfaces by well-defined chemical means1. In particular, the last several years have seen passivation and functionalization of semiconductor surfaces to be strategies to realize devices with superior function2 by controlling Fermi level energies, band gap magnitudes, and work functions of semiconducting substrates. Heretofore, functionalization studies have dealt mainly with optimizing chemistry to introduce various groups onto the semiconductor surface. As surface functionalization might bring new states into the mid-gap region of the material, electronic effects should also been considered important consequences of chemical modification, which is especially important for semiconductor nanostructures if electronic properties are to be modulated according to the surface functional group3. Optimization of interfaces in inorganic-organic device systems depends strongly on understanding both the molecular processes that are involved in surface modification, and of the effects that such modification have on the electronic states of the semiconductor. Indeed, surface functionalization has already had significant application in semiconductor nanotechnology for fabricating biosensors, catalysis, biomedical devices, solar batteries, and thin film transistors. For long-term stability, covalent surface-grafting of organics may be preferred over polymer coating or layer-by layer deposition by physisorption. Silanes, thiols, carboxylic acids, and alkenes have suitable anchor groups for grafting to semiconductor surfaces, but they do not always form homogeneous films with a high degree of control of molecular packing density and orientation at the surface, both of which are required for semiconductor technological application. Several


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Scheme 1. Proposed mechanism of SAMP formation of organophosphonic acids on titanium oxide. attachment regimens depend on a particular surface termination: Hydrosilylation or diazonium salt reactions require hydrogen-terminated substrates, and silanization hydroxyl-terminated ones. Organophosphonate chemistry is an important alternative to silanization where hydrolytically stable functional interfaces are required4,5; indeed phosphonates form stable, ordered, selfassembled monolayers (SAMPs) on a variety of substrates. They have been used, for example, as stable interface systems to attach biological molecules to the surface of titanium/titanium oxide6 and silicon oxide7. Organophosphonates do not rapidly homocondense to give P–O–P bonds; monolayers are obtained8. SAMP chemistry is markedly different from silanization, which consumes surface OH groups: It has been proposed9 that the formation of organophosphonate interfaces occurs by a process in which surface OH groups may act as initial sites for bonding, but it does not depend on them, and attachment is not precluded by water contamination (Scheme 1). Thus high surface coverage is possible where the introduction of OH groups can only be partially achieved, or where that is incompatible with other requirements of the semiconductor technology. An overview10 details synthetic routes and deposition methods for phosphonates for surface


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functionalization; it also includes their application to biotechnology, catalysis, coating, chromatography, implant technology, and in vivo imaging. The scope of the review presented herein is to highlight the use of this particular chemistry to obtain functional interfaces with improved properties with regard to applications in nanotechnology and device optimization (Scheme 2). An overview of the most relevant contributions in the field is presented, covering recent investigations on the surface modification of technologically relevant materials, on interface engineering of micro- and opto-electronic devices, on nanostructures synthesis, and bio-interface optimization. 2. NEW TRENDS IN ORGANOPHOSPHONATE-BASED SURFACE MODIFICATION Phosphonic acids of general formula RPO3H2 [where R is an organic component] react with a wide variety of metal salts and oxides; this may proceed by an acid-base condensation, coordination mechanism where the acidic OH groups of the phosphonic acid (pKa ≈ 2) react readily with basic metal oxides to form stable P–O–M linkages (≈ 30–70 kcal mol-1 adsorption energy)11, 12. Because of the strong phosphonate-oxide bond, organophosphonates have been used for decades in both industry and academia to treat surfaces of metals that are terminated with a native oxide, encompassing a range of applications, in particular for anticorrosion coatings13. Indeed, the ease organic layer

O

O

P

O

O

R

R

R

O

P

O

O

P

O

O

device

Scheme 2. Applications of organophosphonate chemistry in interfaces optimization.


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of synthesis of phosphonic acids now enables the design of a rich variety of three-dimensional hybrid frameworks, and the fine-tuning of their properties. Organophosphonate chemistry does not require strictly controlled environmental conditions, and obviating condensation reactions between phosphonic head groups allows for the preparation of true, surface-conforming monolayer films. For all these reasons, organophosphonate chemistry attracts growing interest for nanotechnology applications. Where acid-base condensation is difficult to achieve on oxides because of low substrate basicity, simple immersion or spin-casting may result only in physisorption through H-bonding (≈ 10–20 kcal mol-1 bond energy). Here, annealing the phosphonic acid film at 120−140 °C for several hours may be required, preferably under dry conditions, to achieve covalent bonding14. Hanson, et al., developed the “T-Bag” process (tethering by aggregation and growth) to grow SAMs of organophosphonates on such oxides. This method takes advantage of the organized arrangement of amphiphilic molecules at the liquid/gas interface and the transfer of this arrangement to the substrate surface before annealing to secure covalent bonding. Functionalization of various dielectrics, such as TiO2, Al2O3 or HfO2 is typically done in solution. However, depending on the reaction conditions (temperature, concentration, pH, nature of the solvent) dissolution– precipitation may compete with surface modification, so adjustment of deposition conditions should be taken into account8. Surface monolayer ordering can be affected by the morphology of the substrate, as exemplified by the deposition of alkylphosphonates onto ITO substrates of varying roughness15: Octadecylphosphonates maintain film order on lateral ITO grain dimensions down to ca. 50 nm; when surface roughness is greater than the alkyl chain length (ca. 15 Å) the film structure become somewhat disordered. Functionalization with organophosphonates need not be done in solution or on substantially flat surfaces: SAMPs can be prepared by direct microcontact


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printing16; when terminated with reactive functional groups, they can induce bone growth in pores of a beaded titanium implant17; they can be used to adjust the free pore diameter and surface charge in mesoporous TiO218 or to modify the electrical properties of SiO2 micropillars and pores19. Recent investigations on organophosphonate-based surface functionalization focus mainly on three issues: bonding mode; stability with regard to other chemistries; and applications to oxidefree semiconductors. Theoretical and experimental work addresses the bonding mode of organophosphonic acids on different materials. This can be of crucial importance for technological applications, since the mode of bonding may affect the stability of the organic functional layer, its ordering, thickness, surface coverage, molecular conformation in the resulting organic film, and its overall dipole moment. Phosphonic acids have three potential oxygen-based sites for attachment to a surface, of which two involve replacement of phosphonic acid –OH groups by –OM units on the oxide surface (Scheme 3). The third bond between the phosphonate and the surface depends on intermolecular forces of the phosphonate substituent organic groups, for example, optimization of van der Waals interactions

in

an

alkylphosphonate

monolayer

or

orientation

effects

in

a

perfluoroalkylphosphonate one20 and the Lewis acidity of surface sites. As the Lewis acidity increases, the strength of the interaction between the surface and the third oxygen of the phosphonate also increases, which can make the organic groups have a smaller “tilt” angle with regard to surface normal. For single crystal substrates coordination to the surface can depend on exposed crystallographic planes and on reaction conditions: Wagstaffe et al. have shown that, at low coverage, phenylphosphonic acid adsorbs on the anatase TiO2(101) surface under ultrahighvacuum conditions as a bidentate ligand by deprotonation of both phosphonate hydroxyl groups;


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as the coverage is increased, a mixed

bidentate/monodentate

binding

develops21.

mode

Hotchkiss et al. have shown that octadecylphosphonic acid bonds to

the ZnO

surface

predominantly

in

a

tridentate

fashion, forming dense, wellpacked

monolayers

with

primarily anti conformations of Scheme 3. Sketch of possible binding modes for phosphonic acid molecules on metal oxide surfaces Bonding modes: (a) monodentate; (b) bidentate, with hydrogen bonding to a surface oxide; (c) simple bidentate; (d) tridentate; (e) bidentate with residual hydrogen bonding to a surface –OH group.

the

alkyl

chain22.

Interface

electronic structure has been elucidated by density functional theory

(DFT)

calculations,

which address bonding of organophosphonic acids to ZnO and other technologically relevant transparent conducting oxides (ITO, MoO3). They provide information about distributions of bonding modes, possible interconversions among these modes, and distributions in orientation of the film constituent molecules with respect to the surface normal (which defines the z-component of the molecular dipole moment, and thus directly impacts work-function modification)23. Solidstate nuclear magnetic resonance (NMR) has emerged as a method complementary to FTIR and XPS to characterize phosphonate bonding modes. In this study multinuclear (1H,

29

Si,

31

P)

quantitative solid-state NMR measurements showed that geminal silanols on silica nanostructures are preferentially depleted through functionalization with the phosphonic acids, and that


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methylphosphonic acids are bound preferentially compared to phenylphosphonic acids, likely because of the slightly differences in acidity between the two phosphonic acids24. Stability of organophosphonate interfaces,

especially on technologically important

semiconductor surfaces has been studied4, 25, 26. Generally, organophosphonate monolayers are stable in acidic, neutral, and physiological solutions; under basic conditions breakdown of the monolayer may be observed, as on steel27, silica, titanium or mica28. This may be attributed to the solubility of the native oxide coating under alkaline conditions, which would also cause dissociation of the phosphonate, inter alia, from the substrate surface; a systematic investigation of this proposal has not yet been performed. It has been shown that octadecylphosphonate binds to sputter-deposited zinc oxide (ZnO) films predominantly in a tridentate fashion, forming dense, well-packed monolayers with alkyl chains in fully anti conformations22. However, phosphonic acids can etch ZnO29, and have been shown to precipitate and give cluster formation on singlecrystal ZnO(101̅0) surfaces30. Monolayer deposition on ZnO can be achieved by careful control of the deposition conditions31, and a SAMP grown on ZnO was shown to provide better corrosion resistance against a Brønsted acid than did a comparable alkanethiolate SAM32. Studies on TiO2 anatase (101) and rutile (110) surfaces show that phosphonates bind more strongly to TiO2 than do carboxylate analogs. Strong interfacial electronic coupling between the orbitals of the adsorbed phosphonates and the electronic states of the anatase (101) surface slab leads to new states in the band gap of the otherwise pristine surface; for rutile (110) no, or only weak, coupling occurs between these orbitals and surface states at the band edges33. These results are partially confirmed by a DFT study of the adsorption of n-butylphosphonic acid on the anatase (101) TiO2 model surface, where a work function decrease by 0.7 eV is predicted for anastase when a full selfassembled monolayer is present on the surface34. Phosphonic acids have been proposed as TiO2


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anchoring groups to improve device stability in dye-sensitized solar cells; a systematic comparison between carboxylic and phosphonic groups show that they perform similarly, but the stability in aqueous media is significantly higher for organophosphonate anchoring groups35, 36. Alkyl- and perfluoroalkylphosphonate self-assembled monolayers have good stabilities in PBS buffer on hafnium and aluminum oxide surfaces that were deposited by atomic layer deposition (ALD); the phosphonate SAMs provide longer-term stability under aqueous conditions than do similar silanebased monolayers37. An important consideration for nanotechnology is to achieve controlled surface modification of oxide-free semiconductor surfaces, where methods may rely, for example, on reactions of H-terminated surfaces by appropriately functionalized organics, such as olefins for coating by hydrosilylation, or by substitution with other chemical species (e.g. halogens). Other functional groups present on the surface (e.g. OH or NH2) can be used as anchoring groups for further reaction. In this way, ordered layers of monodentate phosphonates have been grown on oxide-free surfaces, such as H-terminated Si(111)38, 39 or 6H-SiC40. Interestingly, Thissen et al. have shown that phosphonic acids on oxide-free Si(111) are monodentate-bonded to the surface; the interface has high electrical quality (a low density of electronic interface traps), and remarkable stability38. A similar approach has been applied successfully on SiC to engineer work function properties of this wide-band gap semiconductor, on which smooth and pinhole-free monolayers of alkyl- and arylphosphonates were formed40.

3. APPLICATIONS OF ORGANOPHOSPHONATE FUNCTIONAL INTERFACES


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Advances in nanotechnology require controlling and tuning materials properties, and it is not surprising that surface modification using organophosphonate chemistry is an attractive technique that can capitalize on the versatility of these species for controlled grafting at the nanometer scale. As highlighted by Bujoli et al.10 control of the density and orientation of surface organophosphonate components can be achieved by careful attention to deposition and annealing conditions; detailed characterization protocols are also required. Nonetheless, a substantial fraction of the reports of the last few years focuses more on possible applications of this chemistry, and often with insufficiently attention paid to clear experimental protocols to enable optimization and reproducibility of the prepared functional interfaces. It is even unclear in some cases if molecular species on the surface are merely physisorbed as phosphonic acids or are instead covalently bound to it as phosphonates. Perhaps for this reason the same materials, following derivatization with the same organic species, may show differing behavior with regard to tunable properties in diverse systems. Still, the richness of application of phosphonate methodology is readily and amply documented as described herein. 3.1.

Organophosphonate-based interface engineering in organic thin-film

transistors. Self-assembled monolayers can be used to modify the gate dielectric/semiconductor interface in organic thin-film transistors41. In this way, interactions between the molecular semiconductor and the substrate, ordering in the thin film, and electronic properties of the semiconducting channel can be controlled, which results in macroscopically observed changes in charge-carrier mobilities, threshold voltages, subthreshold swing, and transfer characteristic hysteresis; the latter two are determined by the density of charge-trapping states at the interface (Figure 1). In one striking example42 of the use of SAMPs to modulate interfacial structure in ultrathin film organic field-effect transistors (OFETs) using pentacene as the active channel11, 12, 43-46 ,


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transport properties were substantially improved through designed phosphonate spatial ordering at the interface, which was attributed to enhancing pentacene orbital overlap in the semiconductor layer. OFETs based on substituted oligothiophenes47-49, solution-processed polymer50, C(60)12, 45, rubrene active channels23, and perylene diimide51 have also been reported. SAMP-modified gate dielectrics have been used for threshold-voltage control in low-voltage OTFTs. Multifunctional SAMs formed on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, enable low-voltage OFETs through dielectric and interface engineering. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes and contact resistances and high charge carrier mobilities52. It is interesting that fluoroalkylphosphonate SAMPs formed on patterned, plasma-oxidized aluminum gate electrodes cause a change in the transistor threshold voltage by about 1 V, which is attributed to the strong electron-withdrawing character of the fluoroalkyl chains. These observations have

Figure 1. Schematic of SAM/metal oxide hybrids (a) and typical organophosphonate interface (b) with optimized dielectric properties for low-voltage OFETs. Reprinted with permission from ref. 43. Copyright 2012 Royal Society of Chemistry.


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been discussed in a report on the “Optimum Combination of Semiconductors and Monolayer Gate Dielectrics” for organic semiconductor thin film devices

53

. Organophosphonic acids were

employed beneficially in the fabrication of multilayer materials as versatile gate dielectrics with excellent thermal stability for low operating voltage organic and inorganic semiconductor-based TFT devices. Here, multilayer dielectrics using phosphonate-based organic and zirconium-based inorganic precursors affords robust, smooth, adherent, insulating, pinhole-free, high-capacitance, thermally stable, ultrathin dielectric materials 54. The effect of the alkyl chain length in SAMP-modified gate dielectrics on the performance and stability of low-voltage organic thin-film transistors has been carefully examined55; for pentacenebased OTFTs there is a pronounced effect on performance with increasing chain length, and optimum performance occurs with chains between C8 and C1456, 57. Combined experimental and computational studies of SAMPs of n-alkylphosphonates ranging in chain length from C6 to C18 that were prepared on Al2O3 for study in large area thin film devices (OFETs with good hole mobilities up to 0.3 cm2 /Vs at -1 V) and capacitors have shown that chain length dependence is not consistent with theoretical descriptions of tunneling through saturated n-alkanes58. An unexpected saturation in the current density was measured for devices incorporating SAMPs of increasing chain length, which also affected the leakage current in transistors and the current density in capacitors. These effects are attributed to differing morphologies, ranging from an amorphous state for phosphonates with short alkyl chains (for example, C6), to a quasi-crystalline one with longer alkyl chains (C14-C18). Molecular dynamics (MD) simulations provide insight into the nature of three-dimensional intermolecular interactions in these films; they support the hypothesis that changes in morphology can lead to a reduction in the effective SAM thickness in devices, which can be described quantitatively through a Simmons approach. These conclusions


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have been substantiated through study of six SAMPs with tailored molecular structures that are used as dielectric modifying layers: SAMPs that contain bulky terminal groups, or that are highly crystalline in nature, do not yield homogenous surface functionalization at the molecular level, and result in low charge carrier mobilities in devices in which they are present59 . High saturation fieldeffect mobilities result from a balance between disordered SAMs that promote large pentacene grains and thick SAMs that aid in physically buffering the charge carriers in pentacene60. A putative relationship between SAM surface energy and charge carrier mobility is controversial: Novak et al.61 observed that high surface energy SAMs tend to induce face-on growing of the ,dihexylquaterthiophene semiconductor in thin film transistors, whereas low surface energy SAMs favor edge-on growth; Liu at al.42 developed high surface energy SAMPs to enhance solutionprocessed n-channel OTFTs that have average field effect mobilities of up to 2.5 cm2/Vs. In contrast, Hutchins et al.59 observed no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices. Aromatic SAMPs fabricated on SiO2 surfaces are reported to influence crystallization of vapor deposited pentacene, and thus to affect device performance of pentacene-based organic thin film transistors62,

63

. Vibrational spectroscopic

characterization of pentacene constituents in these films revealed that the molecular orientation of adjacent crystalline grains is strongly correlated on the SAMP-modified dielectric surface, which results in enhanced interconnectivity between the crystallite domains; vibrational coupling interactions, relaxation energies, and grain size boundaries also varied in pentacene thin films with the choice of SAMP. There is a reduction in the channel current when OFETs are subjected to continuous gate bias. The origin of this stress instability is variously attributed to mechanisms involving charge trapping in the semiconductor layer or at the interface, trap formation within the dielectric, ionic displacements


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in the dielectric, or structural changes in the semiconducting material. For low-voltage OFETs based on ultra-thin SAMPs, dielectric threshold voltage instabilities are particularly important, and have been studied accordingly64. Yet, charge trapping in localized states in the semiconductor does not seem to be the sole mechanism responsible for this observed instability; terminal functional groups of the SAMP constituents, and possible electrochemical processes that occur at the dielectric/SAM/semiconductor interface, may also play important roles. SAMPs have been used to improve both positive and negative bias-stress stability of amorphous indium gallium zinc oxide (IGZO) bottom gate thin film transistors65. These effects were attributed to a reduction in molecular adsorption of contaminants on the IGZO back channel surface, and to only a small number of trapping states that remain when the SAMP bonds to the IGZO surface. 3.2.

Electronic

transport

through

organophosphonate

monolayers.

Alkylphosphonate SAMPs have been used as model system for studying electronic transport through molecular-thick films on SiO2/Si66, 67 and on AlOx surfaces68. In particular, it has been shown that high-quality organophosphonate monolayers constitute an excellent model system for the investigation of electronic transport through monolayer-SiO2 interfaces; at the same time, they also provide highly efficient electrical passivation of the SiO2/Si surface66. Direct tunneling in alkylphosphonates has been characterized at small bias by a decay constant in the typical range for dense monolayers, β ≈ 0.7/C, on both substrates; a higher value, βbis = 1.40/C, was measured for SAMPs made from ,-diphosphonalkanes, which are characterized as having distal phosphonic acid units and less dense molecular packing than their aliphatic analogs, and was attributed to a transport mechanism having a strong contribution of

'through-space' tunneling and direct

interactions of the distal phosphonic acid groups68. SAMPs made from ,-diphosphonarenes had significantly high capacitances, 7 - 10 μF/cm2, as measured by impedance spectroscopy


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characterization, consistent with higher polarizability of these aromatic monolayers versus prepared aliphatic analogues of similar length69, 70. Self-assembled organophosphonate duplex ensembles were also synthesized on nanometer-thick SiO2-coated, highly doped silicon electrodes. They were prepared by first treating the SAMP from an aromatic diphosphonic acid with a titanium tetra-alkoxide to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. The duplex was compared with aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties, and data were analyzed using resistance– capacitance

network-based

equivalent circuits. For anthracenebased SAMPs, CSAMP = 6–10 μF/cm2, impedance spectroscopy Figure 2. Chemical structure of a phosphonic acid unit, with a conducting perylene core (a) used to fabricate self-assembled monolayer field-effect transistors for flexible organic electronics (b). Reprinted with permission from ref. 73. Copyright 2013 Elsevier.

measured the additional capacitance of the second aromatic monolayer in the duplex to be CTi/2ndSAMP = 6.8 ± 0.7 μF/cm2, in series with the first monolayer70.

Top-contact

self-

assembled monolayer field-effect transistors (SAMFETs) were fabricated by solution assembly, and had higher conductance using SAMPs of diphosphonoanthracene than of alkyl C16-SAMPs71. Organophosphonate-based SAMFETs can also be fabricated by spin-coating in channel lengths between 12 - 80 µm, and show


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reproducible field-effect mobilities of 1.1-8 x 106 cm2/ Vs72. The first examples of n-type SAMFETs have been prepared on flexible substrates from aliphatic SAMPs in which a perylene3,4,9,10-tetracarboxylic dimide unit is incorporated into the alkyl chain; they exhibited electron mobilities in the range of 10−3 cm2/Vs73, 74 (Figure 2). Studies of SAMFET devices comprising fullerene-functionalized self-assembled monolayers that were mixed with alkylphosphonic acids of various chain lengths showed that high 2D crystalline order is not the only important consideration for transport: If the fullerene head groups are confined by the supporting alkylphosphonate molecules, defects in the crystalline C60 film such as grain boundaries can strongly limit charge transport properties75. 3.3.

Incorporation of organophosphonic SAMs into ITO light emitting diodes and

other microelectronic devices. Indium tin oxide (ITO) is widely used as transparent electrodes for layered devices such as liquid-crystal displays, organic light-emitting devices, and solar cells. Barriers to charge-carrier injection from ITO into an organic layer can result from unfavorable energy alignment factors, and poor wetting of the hydrophilic ITO by these typically hydrophobic layers can compromise device function. ITO surface modification by organics provides the means to address these issues76, as had been done with phosphonates for engineered AlOx heterojunctions in polymer light emitting diodes77. The electronic structure and geometry of attachment of molecular constituents at the interface between the ITO and an organic layer are critical for device function, and in this context SAMPs have been shown to yield beneficial interfacial layers between an ITO electrode and an organic carrier transport layer78-81; for example, they can adjust the work function of ITO from about 4.25 eV to about 5.40 eV82,

83

. In particular, for

pentafluorobenzylphosphonic acid the increased of the ITO work function observed from 4.6 eV


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to 4.95 eV, resulted in an increased power conversion efficiency of 4.2 % in a solar cell, which was attributed to improved hole extraction84.

Figure 3. Changes in built-in potential across a photoactive layer in diodes by systematically modifying the indium tin oxide electrodes with dipolar organophosphonate SAMPs. A linear regression is shown versus the mean work function of ITO/SAM surfaces with the magnitude of the slope, S. Reprinted and adapted with permission from ref. 87. Copyright 2012 American Chemical Society. Detailed studies quantitatively correlate the normal component of the dipole moment of surfaceattached species precursors and the molecular loading per unit area to measured changes in the work function for phosphonates on ITO or conducting polymers83, or at organic–organic semiconductor heterojunctions85. Most reports, however, describe such correlations only qualitatively. Nonetheless, they provide firm evidence for the effect of ITO surface modification with these materials, several examples of which follow. The change in built-in potential (VBI) across a polymer photoactive layer in diodes was investigated by systematically modifying the indium tin oxide electrodes with dipolar SAMPs86, 87 (Figure 3). The enhanced performance of


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devices incorporating these modifiers was related to improved charge injection that resulted from enhancing wetting of the hydrophilic ITO by the hydrophobic organic hole transport layer88-90. It was proposed that SAMPs may serve to reduce atmospheric/process contamination prior to spincoating a polymer, and that the relatively thin SAM layer does not inhibit interfacial electronic overlap between electronic states of the ITO and those of the organic. Conducting-tip atomic force microscopy of ITO heterojunctions shows that, indeed, SAMP modification of the ITO surface produces a uniform and ohmic response, prevents the buildup of a contamination layer, and enables charge harvesting through this insulating layer with minimally deleterious impact on device performance91. It is interesting that SAMPs can be patterned on ITO by microcontact printing to achieve surface contact potential difference with sharply defined edges and with large work function contrasts92. Comparable electroluminescent properties in devices fabricated by this technology versus solution-soaked SAM devices indicate large, consistent changes in charge injection across the microcontact-printed surfaces. 3.4.

Engineering at ZnO Organic Interfaces with Phosphonate-Based Self-

Assembled Monolayers in optoelectronic applications. SAMPs can engineer energy level alignment at hybrid inorganic/organic ZnO interfaces. Zinc oxide (ZnO) is a wide-band gap II-VI semiconductor; it has optoelectronic applications because of its transparency to visible light and its tunable optical/electronic properties that can be achieved by doping. However, ZnO suffers from poor chemical stability. ZnO also has a low work function, which results in a substantial barrier for charge injection into commonly used organic semiconductors that constitute the active layer in a device. Surface organophosphonates can be used to improve the chemical stability of ZnO, to adjust its work function93, 94, and to affect the interface with an organic semiconductor. In particular, SAMPs can be used to adjust the surface energy and improve contact between vertically


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oriented ZnO nanowire arrays and the oligothiophene (P3HT) semiconductor that are incorporated into hybrid organic-inorganic solar cells31. Both surface passivation of the ZnO and the increased wettability of the ITO by the organic phase improve charge transfer and reduce carrier recombination at the organic-inorganic interface in the device. ZnO-oligophenylene heterojunctions can be engineered using substituted arylphosphonates to allow for control of molecular dipole moments; the interplay between SAMP-induced work function modification and changes in oligophenylene orientation enabled control over offsets between molecular frontier energy levels and semiconductor band edges95. Similar work function control has been achieved in oligothiophene-based inverted solar cell structures, which demonstrates the use of SAMPmodified ZnO as either electron or hole extracting electrodes in hybrid optoelectronic devices96. Improved photocurrent in hybrid photovoltaic cells based on SAMP-modified ZnO nanoparticles and P3HT has been related using quasi-steady-state photoinduced absorption spectroscopy to the formation of long-lived polarons at the interface97. Enhanced photovoltaic performance has also been observed in CdS quantum dot-sensitized solar cells, in which electron-hole recombination was suppressed by modifying the ZnO photoanode with SAMPs98. Similarly, Ga-doped ZnO showed an increase in the density of states and changes in the valence band edge characteristics following SAMP deposition99. 3.5.

Assembly of phosphonic acids on GaN and AlGaN. Unlike semiconductor

species such as Si, III-V semiconductors do not readily form a protective layer of oxide, which is problematic both for stability and for situations requiring high K dielectric properties. Surface functionalization with phosphonates could be effective here to stabilize an oxide termination, to alter surface electrical and chemical signatures, and to produce quantifiable changes in optical properties of these materials. In this context, Wilkins et al. studied the stability and the optical


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properties

of

planar

and

nanostructured

GaN

substrates

functionalized

with

perfluoroalkylphosphonates100, 101. Grafting the organophosphonate to the oxidized GaN(0001) surface increased its stability in buffer, acidic, or neutral solutions102. SAMPs can be grown on an aluminum gallium nitride (AlGaN)/GaN heterostructure as well; XPS and UPS analyses and conductivity measurements made before and after molecular deposition indicated that the channel conductivity was not affected by the molecular dipoles created on the surface, most likely due to a high concentration of surface states pinning the Fermi level103. Clearly, though, these few studies demonstrate the potential of organophosphonate chemistry for passivation and stabilization of IIIV semiconductors in, for example, aqueous environments of surface-sensitive devices. 3.6.

Organophosphonate

capping

ligands

for

nanostructures

synthesis.

Organophosphonates can be effective capping agents for use in the synthesis of nanostructures. Size and shape evolution, from pseudo-spherical silicon nanocrystals to well-faceted tetrahedralshaped silicon crystals with edge lengths in the range of 30-400 nm, can be induced through sequentially decreasing the chain length of alkylphosphonic acid surfactants104. These surfactants enable the synthesis of ZnO nanorods with highly adjustable shapes, lengths (40-200 nm), diameters (6-80 nm), and doping levels105, 106. CdSe quantum structures of various shapes can be synthesized reproducibly using cadmium-phosphonic acid complexes as precursors107,

108

.

Bonding energies of and steric hindrance imparted by phosphonate ligands can dramatically affect both the growth kinetics of CdSe nanocrystals and their resulting geometries109. In particular, the chain length of alkylphosphonate ligands affects the equilibrium between wurtzite and zinc blende polytypes of CdSe nanocrystals110. It also has an impact on the morphology of CdSe nanorods prepared by colloidal synthesis; the higher the mole fraction of the shorter alkyl group, the more elongated and branched are the nanorods111. Colloidal CdS nanorods with diameters from about 4


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nm up to 300 nm can be synthesized, for example, by sequential reactant injection utilizing phosphonic acids as capping ligands; they strongly passivate the nonpolar CdS surfaces, and sequential reactant injection provides for controlled CdS formation kinetics to enable heterogeneous and facet-selective CdS deposition on the more reactive surface. In this way, nanorod lengths can be increased controllably by reactant addition112 (Figure 4). Changing the capping agents may control the morphology and phase of colloidal cadmium telluride (CdTe) nanocrystals as well. Controlling the morphology and phase of colloidal CdTe nanocrystals can be achieved using alkylphosphonates having chain lengths ranging from C 4 to C18; the largest tetrapods can be produced using decylphosphonic acid113. Organophosphonate

Figure 4. Transmission electron micrographs of CdS nanorods (a−d) obtained at different times during the growth process, by a sequential reactant injection technique that utilizes phosphonic acids as capping ligands. Reprinted with permission from ref. 112. Copyright 2008 American Chemical Society.


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surfactants mediate electron transfer between nanocrystals and organic semiconductors114; as ligands they play a fundamental role in controlling photoemissive properties of CdSe and CdTe nanostructures115-118 and in enhancing the stability of Y2O3 in aqueous media119. 3.7.

Molecular engineering of 1D/2D and spintronic materials. Self-assembled

monolayers can dope 2D materials such as graphene. SAMs deposited on various dielectrics can act directly as doping agents, or can influence charge mobility by limiting the interactions between the graphene and substrate surface trap states. In this context, SAMPs, with their high packing densities, would have a stronger doping effect that do alkylsilane-based SAMs; in fact these alkylsilanes had little to no effect for doping graphene transistors120. SAMPs prepared from arylphosphonic acids are suitable for interfacial engineering of the (opto-) electronic properties of atomistically thin sheets of semiconducting transition metal dichalcogenides; the charge carrier density of a MoS2 monolayer can be depleted by about Δn  7 × 1012 cm-2 on a SAMP-treated substrate compared to planar SiO2 or sapphire substrates. This finding points towards a depletion of intrinsic n-type doping of MoS2 membranes by aromatic SAMPs, and clearly indicates that organophosphonate interfacial chemistry can play an important role in controlling the performance of 2D materials121. Organophosphonates can reduce thermal contact resistance between carbon nanotubes and metal oxide surfaces; these surface modifiers facilitate a roughly 9-fold reduction in the thermal contact resistance over the dry contact when used to bond nominally vertically aligned multi-walled CNT forests to Cu oxide surfaces122. SAMPs can be used to tune the wettability of the surface and work function of spintronic devices. With only a few reports in the literature here, organophosphonates have already shown their potential as effective anchoring groups for (La,Sr)MnO3 manganite (LSMO), which is one of the most widely used ferromagnetic electrodes in spintronic devices123 (Figure 5); it is significant that


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SAMP grafting does not affect electrode spin polarization properties of this material, which clearly highlights the potential of such SAMPs on LSMO in the development of molecular spintronic devices.

Figure 5. Sketch of deodecyl phosphonate molecules (C12P) on (La,Sr)MnO3 manganite and representative I(V) traces at T = 4 K of Co/C12P//LSMO junctions in the aligned (black) and opposite (gray) magnetic configurations. The difference in device conductivity demonstrates that the LSMO surface is still spin polarized. Reprinted and adapted with permission from ref. 123. Copyright 2012 American Chemical Society.

3.8.

Application of organophosphonate surface functionalization for bio-interface

optimization. SAMP biointerfaces are usually prepared stepwise involving a base monolayer that is elaborated with biological recognition ligands. The surface areal density of such ligands can be controlled using ratios of two different phosphonates where only one has a terminal group that is compatible with further coupling. Selective cell adhesion studies or protein immobilization can confirm successful surface biofunctionalization, as shown for biointerfaces developed on ITO124 or on porous aluminum oxide125. SAMP-based devices were derivatized with PEG-modified DNA


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and DNA analog PNA oligomers; this method holds promise for smooth integration of SAMPs and semiconductor technology for the fabrication of DNA-based microdevices126,

127

. SAMP

monolayers are especially well suited for biosensing applications due to their mild preparation conditions and their facility for chemical amplification using simple, water-based conjugation chemistry; they are particularly indicated where mild conditions are demanded for functionalization with sensitive detection moieties. Here, SAMPs can be grown on silicon nanowires that are integrated into a chip for label-free detection of DNA hybridization128, or on photonic microring resonators for label-free detection of glycan−protein and glycan−virus interactions129, or on porous aluminum oxide for bio-immobilization125. SAMPs have been used as linkers to tether biomolecules onto OH-terminated diamond surfaces130, and SAMPs can function as stable platforms to interface with redox-proteins that form fundamental building blocks for many bioelectronics applications131. SAMPs can promote cell adhesion, and for this reason have found application in implant technology9,

132-134

. They are stable on immersion under

physiological-like conditions6, 135, 136 and, unlike phosphate groups, phosphonate moieties are not readily hydrolyzed enzymatically in a biological environment137; together, these features makes SAMPs attractive as bio interfaces. Organophosphonates can be bound at the nanoscale to polymeric nanoparticles138 and to poly(arylether-ether-ketone) (PEEK), which is emerging as a material for use in trauma, orthopedic, and spine applications; in the latter case PEEK functionalization is achieved through an oxide-based adhesion interlayer, and - as so derivatized - yields surfaces that are significantly more active for cell attachment and spreading than is untreated PEEK139. Even more complex biopolymer surfaces have been prepared on PEEK: The adhesion interlayer can be applied by chemical vapor deposition of a Zr alkoxide through a photolithographic pattern followed by thermolysis. After selective


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biofunctionalization using SAMP linkers, aligned cell proliferation occurs across an entire twodimensional surface140, 141 (Figure 6). As a last word, phosphonic acids and their derivatives are

R R P P O OO O O O O OZr O Zr O O O O O OZr O Zr

(a) Photolithographically

pattern (b) Zr(OBut) 4

R R P P O OO O O O O OZr O Zr O O O O O OZr O Zr

HO R (c) HO P O O O O O O OZr Zr O OZr O Zr O O O OO O OO O O O OZr O Zr O OZr O Zr O O O

Heat

(d)

Figure 6. Schematic for the preparation of a nanoscale-patterned surface using a self-assembled monolayer of phosphonates (SAMP). (a) The substrate is patterned photolithographically using standard methods; (b) a Zr tert-butoxide is evaporated onto the surface, and adheres in exposed regions; it is then heated to thermolyze the tert-butoxy ligands to give surface-attached ZrO2; (c) a cell-adhesive SAMP is formed where the ZrO2 is present, as shown by cell alignment with the pattern (d); Reprinted and adapted with permission from ref. 140. Copyright 2013 Royal Society of Chemistry.


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receiving attention as analogues of naturally occurring phosphates and as "bio-isosteric phosphorus analogues" of amino acids.

4. CONCLUSIONS AND OUTLOOK In this review, we have focused on the relevance of organophosphonate chemistry in nanotechnology, giving an overview about some recent applications of this class of monolayers for the preparation of novel interface systems with crucial relevance in micro- and opto-devices, in nanostructures synthesis and modification, and bio-interface optimization. From our examination of the published work in the field, it is evident that the phosphonic acid group is one of the most highly versatile functional groups available for achieving control of the interface properties at molecular level. Organophosphonate interfaces show improved stabilities and adaptability in electronic devices and biosensing applications in respect to other strategies available for the integration of organic films in technology, enabling the implementation of surface modification schemes into real fabrication processes. Moreover, they can be easily applied to a broad class of semiconductor and dielectric surfaces, with applications on nanostructures and emerging materials as well. Among all the methods available for grafting of phosphonates onto surfaces, the T-Bag method has been found to be reliable to achieve a high degree of control of molecular packing density and orientation at the surface, both of which are remarkably important for the systematic investigation of physicochemical phenomena at the interface. In this sense, we have highlighted in this overview several of the recent reports in the literature, which focus on a detailed experimental and theoretical characterization of such interfaces towards a better understanding of the parameters, which govern the grafting and rearrangement of organophosphonates on surfaces. This is of particular importance also in applications, like OFETs,


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in which organophosphonate chemistry has found large use since decades, showing, however, some contradictory results, probably because of insufficient attention paid to clear experimental protocols to characterize the prepared functional interfaces. We have highlighted also some more recent functionalization studies focusing on the modification of oxide-free semiconductor surfaces and on the growth of organized multilayered and multicomponent hybrid systems because of their particular importance in nanotechnology. The combination of organophosphonate monolayers and patterning techniques to build devices with chemical addressability onto small features has also been briefly discussed, since integration of different complex structures at the nanoscale is of great interest in nanotechnology. Especially for biosensor miniaturization and biomimetic platform optimization, we believe that organophosphonate chemistry will play a crucial role in developing new functional interfaces of high biocompatibility and applicability. The richness of applications reviewed in this work already and clearly show the potentiality of this approach to support the development of groundbreaking technologies.

AUTHOR INFORMATION Corresponding Author: Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany. E-mail: [email protected] ACKNOWLEDGMENT We thank Jeffrey Schwartz (Princeton), Marc Tornow (TU Munich), and Martin Stutzmann (TU Munich) for helpful discussions. The author acknowledges funding by the DFG (grants CA 1076/3-2, CA 1076/5-1, and CA 1076/6-1).


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