Organic thin-fi lm transistors (OTFTs) [ 1 ] are compatible with large area fabrication techniques, and they may yield new paradigms for manufacturing that would reduce device costs for large area circuits. [ 2 ] Pentacene-based OTFTs are well studied; yet they can be compromised with regard to critical device parameters: on/off ratios, carrier mobilities, sub-threshold performance, and threshold voltages. Surface treatments for the SiO 2 gate dielectric [2][3][4][5][6][7][8] can be effective; typically, however, improvement is realized in only one of these critical areas, often to the detriment of the others. [ 5 , 6 , 9-11 ] Here we show that systematic structural modifi cation of self-assembled monolayers of phosphonates (SAMPs) fabricated on SiO 2 gate dielectrics can yield pentacene-based devices in which all four critical parameters are enhanced simultaneously. We hypothesize that phosphonate structural motifs enable two-dimensional SAMP surface coverage to translate to a three-dimensional one through controlled lateral spacing between vertically oriented acene units, which can affect crystallization of vapor-deposited pentacene.Our approach to surface modifi cation of the SiO 2 gate dielectric on a Si device is based on a simple procedure in which a dilute solution of a phosphonic acid is drawn down across the face of the Si substrate. This process, nicknamed the T-BAG, [ 12 , 13 ] enables formation of high quality SAMPs on these substrates under mild conditions. In previous work we showed that a surface chemistry design approach to transistor function optimization based on T-BAGged SAMPs, progressing from simple enhancement of surface wetting to inducing more specifi c SAMP-pentacene interactions, enabled systematic improvement in OTFT behavior. [ 14 , 15 ] Our best-performing OTFT was based on a SAMP of 9-phosphonanthacene ( 1 ). Anthracenebased phosphonates are electronically acceptable for SiO 2 gate dielectric modifi cation: Their SAMP constituents should have a band gap larger than that of pentacene itself (1.86eV by optical absorption); [ 16 ] thus the SAMP would not create inherent trapping states. Furthermore, an anthracene-based SAMP and pentacene should have only C-H bond-π -system interactions, not stronger bonding ones that could, themselves, create trapping states in the semiconductor gap. Indeed, we found that OTFTs fabricated on 100 nm SiO 2 gate dielectrics terminated with a SAMP of 1 showed high on/off ratios, good threshold voltages, and subthreshold slopes as steep as 200 mV/decade [ 14 , 15 ] in contrast to devices prepared on untreated SiO 2 where subthreshold slopes on the order of about 5 V/decade, and threshold voltages of tens of volts are typical. [ 11 ] Pentacene-based devices fabricated with SAMPs of 1 were also compared to devices made with octadecyl trichlorosilane and with its SAMP analog, octadecylphosphonic acid. These three types of devices all had hole mobilities that were indistinguishable within experimental error. [ 14 , 15 ] Tight molecular packing of 1 on the ...