Photopolymerization of self-assembled monolayers of diacetylenic alkylphosphonic acids on group-III nitride substrates

Article abstract of DOI:10.1021/la100273q

This paper describes the fabrication and characterization of photopolymerizable alkylphosphonate self-assembled monolayers (SAMs) on group-III nitride substrates including GaN and Al_xGa_1-xN (AlGaN; x = 0.2 and 0.25). Contact angle goniometry, visible absorption spectroscopy, and atomic force microscopy were used to assess the formation, desorption, and photopolymerization of SAMs of diacetylenic alkylphosphonic acids (CH₃(CH₂)_n-C≡C-C≡C-(CH ₂)_mPO(OH)₂; (m, n) = (3, 11), (6, 8), and (9, 5)). As with GaN substrates (Ito, T.; Forman, S. M.; Cao, C.; Li, F.; Eddy, C. R., Jr.; Mastro, M. A.; Holm, R. T.; Henry, R. L.; Hohn, K.; Edgar, J. H. Langmuir 2008, 24, 6630-6635), alkylphosphonic acids formed SAMs on UV/O ₃-treated AlGaN substrates from their toluene solutions in contrast to other primary substituted hydrocarbons with a terminal -COOH, -NH ₂, -OH, or -SH group. Diacetylenic alkylphosphonate SAMs on group-III nitrides could be polymerized by UV irradiation (254 nm), as indicated by the appearance of a visible absorption band around 640 nm and also by their significantly reduced desorption from the surface in a 0.1 M aqueous NaOH solution. A longer UV irradiation time was required to maximize the photopolymerization of a SAM having a diacetylene group close to the terminal phosphonate moiety, probably because of the hindrance of the topochemical polymerization due to the limited flexibility of the cross-linking moieties on an atomically rough substrate surface.

Full text of DOI:10.1021/la100273q

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© 2010 American Chemical Society
Photopolymerization of Self-Assembled Monolayers of Diacetylenic
Alkylphosphonic Acids on Group-III Nitride Substrates
Feng Li,† Evgeniy Shishkin,† Michael A. Mastro,‡ Jennifer K. Hite,‡
Charles R. Eddy, Jr.,‡ J. H. Edgar,§ and Takashi Ito*,†
†Department of Chemistry, Kansas State University, 213 CBC Building, Manhattan, Kansas 66506-0401,
‡Code 6880, U.S. Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, D.C. 20375, and
§Department of Chemical Engineering, Kansas State University, Durland Hall, Manhattan, Kansas 66506-5102
Received January 19, 2010. Revised Manuscript Received May 16, 2010
This paper describes the fabrication and characterization of photopolymerizable alkylphosphonate self-assembled
monolayers (SAMs) on group-III nitride substrates including GaN and AlxGa1-xN (AlGaN; x = 0.2 and 0.25). Contact
angle goniometry, visible absorption spectroscopy, and atomic force microscopy were used to assess the formation,
desorption, and photopolymerization of SAMs of diacetylenic alkylphosphonic acids (CH3(CH2)n-CtC-CtC-
(CH2)mPO(OH)2; (m, n) = (3, 11), (6, 8), and (9, 5)). As with GaN substrates (Ito, T.; Forman, S. M.; Cao, C.; Li, F.;
Eddy, C. R., Jr.; Mastro, M. A.; Holm, R. T.; Henry, R. L.; Hohn, K.; Edgar, J. H. Langmuir 2008, 24, 6630-6635),
alkylphosphonic acids formed SAMs on UV/O3-treated AlGaN substrates from their toluene solutions in contrast to
other primary substituted hydrocarbons with a terminal -COOH, -NH2, -OH, or -SH group. Diacetylenic alkyl-
phosphonate SAMs on group-III nitrides could be polymerized by UV irradiation (254 nm), as indicated by the
appearance of a visible absorption band around 640 nm and also by their significantly reduced desorption from the
surface in a 0.1 M aqueous NaOH solution. A longer UV irradiation time was required to maximize the photo-
polymerization of a SAM having a diacetylene group close to the terminal phosphonate moiety, probably because of the
hindrance of the topochemical polymerization due to the limited flexibility of the cross-linking moieties on an atomically
rough substrate surface.
Introduction
for fabricating a pH sensor because of the high affinity of its
surface to OH- and/or Hþ.1,4-7 In addition, the selectivity and
sensitivity of a nitrides-based sensor can be tailored by modifying
the nitride’s surface with a thin layer that selectively recognizes
chemical species of interest.3 For example, organosilane SAMs
with terminal binding moieties were immobilized directly onto the
AlGaN surface in the gate region to control the detection
selectivity of AlGaN-based transistor-type sensors.8-11 However,
formation of thick polymer layers due to the high reactivity of
organosilanes with water and also the hydrolysis of organosilane
SAMs at basic pH12,13 can deteriorate the reproducibility of the
sensor performance. Thiolate SAMs14 were more commonly used
to develop AlGaN-based chemical and biological sensors for
various chemical and biological analytes.3,15,16 These SAMs could
This paper reports the properties of diacetylenic alkylpho-
sphonate self-assembled monolayers (SAMs) on UV/O3-treated,
metal-polar group-III nitride substrates. These SAMs were char-
acterized using contact angle goniometry, transmission visible
absorption spectroscopy, and atomic force microscopy (AFM).
Alkylphosphonate SAMs were formed on an AlxGa1-x
N
(AlGaN) substrate, as with GaN, upon immersion in a toluene
solution of an alkylphosphonic acid. A series of diacetylenic
alkylphosphonic acids were synthesized to study the effects of
cross-linking positions within the SAMs on their photopolymeri-
zation efficiency. Visible absorption spectra suggested that UV
irradiation initially led to the photopolymerization of adjacent
conjugated diacetylene groups in a SAM and then to the degrada-
tion of the cross-linked moieties, which was correlated to the
stability of the SAM in 0.1 M NaOH.
(6) Dimitrova, R.; Catalan, L.; Alexandrov, D.; Chen, A. Electroanalysis 2007,
19, 1799–1806.
Because of their unique electrical properties, low toxicity, and
excellent chemical/physical stability, group-III nitride semicon-
ductors, such as GaN and AlGaN, are employed in chemical and
biological sensors.1-3 These sensors measure changes in their
electrical properties induced by the adsorption of analytes onto
nitride’s surface. Oxide-coated GaN and AlGaN could be used
(7) Tran Ba, K. H.; Mastro, M. A.; Hite, J. K.; Eddy, C. R. J.; Ito, T. Appl. Phys.
Lett. 2009, 95, 142501.
(8) Kang, B. S.; Ren, F.; Wang, L.; Lofton, C.; Tan, W. W.; Pearton, S. J.;
Dabiran, A.; Osinsky, A.; Chow, P. P. Appl. Phys. Lett. 2005, 87, 023508.
(9) Baur, B.; Steinhoff, G.; Hernando, J.; Purrucker, O.; Tanaka, M.; Nickel, B.;
Stutzmann, M.; Eickhoff, M. Appl. Phys. Lett. 2005, 87, 263901.
(10) Petoral, R. M., Jr.; Yazdi, G. R.; Lloyd Spetz, A.; Yakimova, R.; Uvdal, K.
Appl. Phys. Lett. 2007, 90, 223904.
(11) Yakimova, R.; Steinhoff, G.; Petoral, R. M., Jr.; Vahlberg, C.; Khranovskyy,
V.; Yazdi, G. R.; Uvdal, K.; Lloyd Spetz, A. Biosens. Bioelectron. 2007, 22, 2780–
2785.
(12) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-
Blodgett to Self-Assembly; Academic Press: London, 1991.
(13) Ulman, A. Chem. Rev. 1996, 96, 1533–1554.
*To whom correspondence should be addressed. E-mail: ito@ksu.edu.
Telephone: 785-532-1451. Fax: 785-532-6666.
(1) Steinhoff, G.; Purrucker, O.; Tanaka, M.; Stutzmann, M.; Eickhoff, M. Adv.
Funct. Mater. 2003, 13, 841–846.
(2) Chaniotakis, N.; Sofikiti, N. Anal. Chim. Acta 2008, 615, 1–9.
(3) Pearton, S. J.; Ren, F.; Wang, Y. L.; Chu, B. H.; Chen, K. H.; Chang, C. Y.;
Lim, W.; Lin, J. S.; Norton, D. P. Prog. Mater. Sci. 2009, 55, 1–59.
(4) Steinhoff, G.; Hermann, M.; Schaff, W. J.; Eastman, L. F.; Stutzmann, M.;
Eickhoff, M. Appl. Phys. Lett. 2003, 83, 177–179.
(5) Alifragis, Y.; Konstantinidis, G.; Georgakilas, A.; Chaniotakis, N. Electro-
analysis 2005, 17, 527–531.
(14) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M.
Chem. Rev. 2005, 105, 1103–1169.
(15) Wang, H.-T.; Kang, B. S.; Chancellor, T. F., Jr.; Lele, T. P.; Tseng, Y.; Ren,
F.; Pearton, S. J.; Johnson, W. J.; Rajagopal, P.; Roberts, J. C.; Piner, E. L.;
Linthicum, K. J. Appl. Phys. Lett. 2007, 91, 042114.
(16) Kang, B. S.; Wang, H.-T.; Ren, F.; Pearton, S. J. J. Appl. Phys. 2008, 104,
031101.
Langmuir 2010, 26(13), 10725–10730
Published on Web 06/04/2010
DOI: 10.1021/la100273q 10725

Article
Li et al.
Scheme 1. Synthesis of Diacetylenic Alkylphosphonic Acids (1a-c)
be immobilized on the gold-coated gate of an AlGaN-based
transistor by simpler procedures without multilayer formation.
However, the thingold layer deposited onanAlGaN gate reduced
the detection sensitivity of the transistor-type sensors.
improve the physical and chemical stability of nanoscale molecular
assemblies,21,22 Langmuir-Blodgett (LB) films,12,22-27 and
SAMs.28,29 The polymerization of diacetylenic moieties occurs by
simply irradiating with 254 nm UV light, although it requires well-
ordered molecular packing, which is known as topochemical
polymerization.12,21,22,27 The high chemical stability of polydiace-
tylenes, in addition to the selective photopolymerization at the
UV-irradiated area, made it possible to apply diacetylenic SAMs as
ultrathin photoresists in photolithography.30 Diacetylenic SAMs
with terminal functionalities were prepared to fabricate controlled
multilayers31,32 and also to covalently immobilize chemical and
biological recognition moieties.33 In addition, polydiacetylenes
have unique optical properties, leading to their applications for
optical electronic devices and chemical/biological sensors.22,33,34
The combination of the unique optical and electric properties of
GaN/AlGaN and diacetylenic SAMs may thus provide a means for
developing unique chemical and biological sensing devices.
It was previously reported by us17,18 and other researchers19
that oxide-coated GaN substrates could be directly modified with
SAMs of n-octadecylphosphonic acid (ODPA). The SAM forma-
tion was shown by their large water contact angle (103-109°),17,18
ellipsometric thickness (ca. 2.4 nm),18 XPS spectra,18-20 and FTIR
spectra.19 In our previous reports,18 ODPA SAMs were formed by
soaking a UV/O3-treated GaN in its organic solution regardless of
the GaN polarity or dopant concentration. In contrast, primary
substituted hydrocarbons with terminal -OH, -SH, -NH2, and
-COOH groups did not strongly adsorb onto GaN substrates.18
The weak adsorption of -NH2 and -COOH groups will allow us
to modify GaN surface with organophosphonate SAMs having
these terminal functional groups that can be used as linkers of other
functional moieties such as antibodies. However, ODPA SAMs
desorbed from GaN substrates upon immersion in aqueous solu-
tion, especially in basic solution, probably reflecting the high
solubility of deprotonated ODPA in the solution,18 electrostatic
repulsion between the negatively charged ODPA and substrate
surface,18 and/or the dissolution of the gallium oxide layer.19 The
stability of a SAM in aqueous solution, which is required for its
future applications to chemical and biological sensors, may be
improved by cross-linking molecules in the SAM.12
Experimental Section
Chemicals and Materials. 1,3-Dibromopropane (C3H6Br2;
Alfa Aesar), 1,6-dibromohexane (C6H12Br2; Aldrich), 1,9-dibro-
mononane (C9H18Br2; Aldrich), 1,4-bis(trimethylsilyl)-1,3-buta-
diyne (C10H18Si2; Aldrich), 1-bromohexane (C6H13Br; Aldrich),
1-bromononane (C9H19Br; Aldrich), 1-bromododecane (C12H25Br;
Acros Organics), hexamethylphosphoric triamide (HMPA; MP
Biomedicals), methyllithium-lithium bromide complex (Acros
Organics), n-butyllithium (Acros Organics), trimethylsilylbromide
(C3H9SiBr; Aldrich), potassium fluoride dihydrate (Allied
Chemical), triethyl phosphite ((C2H5O)3P; Aldrich), n-octadecanol
(C18H37OH; Alfa Aesar), n-hexadecyl mercaptan (C16H33SH;
Acros Organics), n-octadecylamine (C18H37NH2; Alfa Aesar),
n-octadecanoic acid (C17H35COOH; Aldrich), n-octadecylphos-
phonic acid (C18H37PO(OH)2, ODPA; Alfa Aesar), N,N-dimethyl-
formamide (DMF; Fisher), and sodium hydroxide (Fisher) were
used as received. Tetrahydrofuran (THF; Fisher) was dried by
distillation from Na. CH2Cl2 was dried by distillation from CaH2.
All aqueous solutions were prepared with water having a resistance
of 18 MΩ cm or higher (Barnstead Nanopure Systems). GaN
epiwafers (GaN layer thickness ∼ 3 μm; on single-side polished
sapphire wafers), Al0.2Ga0.8N epiwafers (AlGaN layer thickness
In this study, we prepared SAMs of diacetylenic alkylphosphonic
acids (CH3(CH2)n-CtC-CtC-(CH2)mPO(OH)2; 1a-1c in
Scheme 1) on GaN and AlGaN substrates and investigated their
UV-initiated polymerization behavior and their stability in basic
solution. These molecules have the same chain length (i.e., m þ n =
constant) and a conjugated diacetylene moiety at different posi-
tions. A conjugated diacetylene was chosen as a polymerization
moiety, because its photopolymerization was widely investigated to
(17) Ito, T.; Forman, S. M.; Cao, C.; Eddy, C. R. J.; Mastro, M. A.; Holm, R. T.;
Henry, R. L.; Hohn, K.; Edgar, J. H. ECS Trans. 2007, 11, 97–101.
(18) Ito, T.; Forman, S. M.; Cao, C.; Li, F.; Eddy, C. R., Jr.; Mastro, M. A.;
Holm, R. T.; Henry, R. L.; Hohn, K.; Edgar, J. H. Langmuir 2008, 24, 6630–6635.
(19) Kim, H.; Colavita, P. E.; Paoprasert, P.; Gopalan, P.; Kuech, T. F.;
Hamers, R. J. Surf. Sci. 2008, 602, 2382–2388.
∼1μm; on double-side polished sapphire wafers), and Al0.25Ga0.75
N
(20) Hughes, W. C.; Koh, S. E.; Augustine, B. H.; Polefrone, J. M. Proc.
Electrochem. - Soc. 2001, 2001-1, 213–217.
(28) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf,
H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050–1053.
(29) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189–193.
(30) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995,
117, 5875–5876.
(31) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117,
3963–3967.
(21) Mueller, A.; O’Brien, D. F. Chem. Rev. 2002, 102, 727–757.
(22) Reppy, M. A.; Pindzola, B. A. Chem. Commun. 2007, 42, 4317–4338.
(23) Tieke, B.; Lieser, G. J. Colloid Interface Sci. 1982, 88, 471–486.
(24) Ostermayer, B.; Vogt, W. Makromol. Chem., Rapid Commun. 1982, 3, 563–
568.
(25) Albrecht, O.; Laschewsky, A.; Ringsdorf, H. J. Membr. Sci. 1985, 22, 187–
197.
(32) Gatebe, E.; Herron, H.; Zakeri, R.; Rajasekaran, P. R.; Aouadi, S.; Kohli,
(26) Ostermayer, B.; Albrecht, O.; Vogt, W. Chem. Phys. Lipids 1986, 41, 265–
291.
(27) Carpick, R. W.; Sasaki, D. Y.; Marcus, M. S.; Eriksson, M. A.; Burns, A. R.
P. Langmuir 2008, 24, 11947–11954.
(33) Okada, S.; Peng, S.; Spevak, W.; Charych, D. Acc. Chem. Res. 1998, 31,
229–239.
J. Phys.: Condens. Matter 2004, 16, R679–R697.
(34) Yoon, B.; Lee, S.; Kim, J.-M. Chem. Soc. Rev. 2009, 38, 1958–1968.
10726 DOI: 10.1021/la100273q
Langmuir 2010, 26(13), 10725–10730

Li et al.
Article
epiwafers (AlGaN layer thickness ∼ 1 μm; on double-side polished
sapphire wafers) were prepared via metalorganic chemical vapor
deposition (MOCVD) using thin (ca. 25-35 nm thick) aluminum
nitride nucleation layers on commercial a-plane sapphire substrates.
All GaN and AlGaN samples were c-plane, that is, (0001), oriented,
metal- (i.e., Ga- or AlGa-) polar, and unintentionally doped. Details
of the growth method are described elsewhere.35
(2; 2.4 mmol) was added. The mixture was stirred for 30 min at
0 °C and then for 2 h at room temperature followed by extrac-
tion with ethyl acetate. The organic layer was washed with
saturated NaHCO3 solution, water, and saturated NaCl solution
and dried over MgSO4, and then the solvent was evaporated. The
crude product was purified by silica gel column chromatography
with 20:1 ethyl acetate/hexane as eluent to give light-yellow liquid.
4,6-Nonadecadiyn-1-phosphonate (4a): Yield 72%. 1H NMR
(CDCl3) δ 4.08 (q, J = 6.4 Hz, 4H, OCH2), 2.24 (t, J = 6.8 Hz,
4H, C-CH2), 1.8-1.2 (m, 30H, CH2 and OCH2CH3), 0.90(t, J =
6.8 Hz, 3H, CH3). 13C NMR (CDCl3) δ 65.39, 65.33, 61.56, 61.50,
32.02, 30.86, 30.71, 29.62, 29.46, 29.41, 29.29, 29.23, 29.06, 28.99,
28.53 (d, J = 6 Hz), 26.57, 25.16, 22.90, 22.60 (d, J = 3 Hz), 19.35,
16.67 (d, J = 6 Hz). IR (cm-1): 2977, 2928, 2854, 1456, 1391,
1248, 1164, 1099, 1058, 1029, 960, 815, 788, 722. 7,9-Nonadeca-
diyn-1-phosphonate (4b): Yield 75%. 1H NMR (CDCl3) δ 4.11
(q, J = 6.8 Hz, 4H, OCH2), 2.23 (t, J = 6.8 Hz, 4H, C-CH2),
1.8-1.3 (m, 30H, CH2 and OCH2CH3), 0.90 (t, J = 6.8 Hz, 3H,
CH3). 13C NMR (CDCl3) δ 65.42, 65.36, 61.59, 61.53, 32.05,
30.86, 30.71, 29.64, 29.47, 29.40, 29.30, 29.24, 29.06, 28.99, 28.53
(d, J = 6 Hz), 26.57, 25.17, 22.87, 22.70 (d, J = 3 Hz), 19.39, 16.70
(d, J = 7 Hz). IR (cm-1): 2979, 2929, 2855, 1457, 1392, 1244,
1164, 1097, 1057, 1031, 960, 818, 786, 702. 10,12-Nonadecadiyn-
1-phosphonate (4c): Yield 62%. 1H NMR (CDCl3) δ 4.10 (q, J =
7.2 Hz, 4H, OCH2), 2.24 (t, J = 6.8 Hz, 4H, C-CH2), 1.8-1.2 (m,
30H, CH2 and OCH2CH3), 0.90 (t, 6.8 Hz, 3H, CH3). 13C NMR
(CDCl3) δ 65.42, 65.37, 61.59, 61.53, 32.07, 30.86, 30.71, 29.64,
29.47, 29.41, 29.30, 29.24, 29.06, 28.99, 28.53 (d, J = 6 Hz), 26.54,
25.15, 22.87, 22.59 (d, J = 3 Hz), 19.32, 16.70 (d, J = 7 Hz). IR
(cm-1): 2979, 2927, 2855, 1465, 1391, 1245, 1164, 1098, 1058,
1030, 959, 824, 788, 723.
Synthesis of Diacetylenic Alkylphosphonic Acids (1a-1c;
Scheme 1). 1H and 13C NMR spectra were measured on a Varian
INOVA 400 Fourier transform NMR spectrometer, and chemical
shifts were reported in δ values in ppm downfield of tetramethyl-
silane. IR spectra were measured on a Nicolet Protege 640
spectrophotometer. Exact MS data were measured at the Mass
Spectroscopy Laboratory, University of Kansas.
(a) Synthesis of Diethyl (ω-Bromoalkyl)phosphonates (2a-
2c in Scheme 1).36 The mixture of triethyl phosphite (10 mmol)
with alkyl dibromide (10 mmol) was heated at 140 °C overnight.
After cooling to room temperature, the reaction mixture was
purified by silica gel column chromatography (elution with pure
CH2Cl2 and then 1:15 petroleum ether/ethyl acetate) to give
colorless oil. Diethyl (3-bromopropyl) phosphonate (2a): Yield
87%. 1H NMR (CDCl3) d 4.10 (q, J = 6.0 Hz, 4H, OCH2), 3.48
(t, J = 6.4 Hz, 2H, BrCH2), 2.15 (m, 2H, CH2), 1.93 (m, 2H,
CH2), 1.36 (t, J = 6.8 Hz, 6H, CH3). 13C NMR (CDCl3) d 61.89
(d, J = 6 Hz), 33.87 (d, J = 19 Hz), 26.15 (d, J = 4 Hz), 24.59 (d,
J = 142 Hz), 16.67 (d, J = 6 Hz). IR (cm-1): 2979, 2929, 2855,
1464, 1458, 1441, 1391, 1246, 1165, 1097, 1057, 1030, 959, 786,
1
645. Diethyl (6-bromohexyl) phosphonate (2b): Yield 89%. H
NMR (CDCl3) δ 4.11 (q, J = 7.2 Hz, 4H, OCH2), 3.41 (t, J = 6.8
Hz, 2H, BrCH2), 1.9-1.4 (m, 10H, (CH2)5), 1.35 (t, J = 7.2 Hz,
6H, CH3). 13C NMR (CDCl3) δ 61.62 (d, J = 7 Hz), 33.97 (d, J =
9 Hz), 32.67 (d, J = 5 Hz), 29.88 (d, J = 17 Hz), 27.67 (d, J = 35
Hz), 25.78 (d, J = 140 Hz), 22.52, 16.70 (d, J = 6 Hz). IR (cm-1):
2980, 2935, 2863, 1457, 1443, 1392, 1369, 1241, 1164, 1098, 1056,
1027, 962, 799, 642. Diethyl (9-bromononyl) phosphonate (2c):
Yield 90%. 1H NMR (CDCl3) δ 4.10 (q, J = 6.8 Hz, 4H, OCH2),
3.41 (t, J = 6.8 Hz, 2H, BrCH2), 1.9-1.3 (m, 16H, (CH2)8), 1.34
(t, J = 6.4 Hz, 6H, CH3). 13C NMR (CDCl3) δ 61.57 (d, J = 6 Hz),
34.27, 32.96, 30.76 (d, J = 17 Hz), 29.36, 29.18, 28.88, 28.30, 26.56,
25.15, 22.60, 16.70. IR (cm-1): 2980, 2929, 2855, 1465, 1458, 1442,
1391, 1369, 1247, 1164, 1098, 1057, 1029, 958, 786, 644.
(b) Synthesis of 1,3-Alkadiynes (3a-3c in Scheme 1).37,38
An ether solution of methyllithium-lithium bromide complex
(6.7 mL; 10 mmol) was added dropwise to a solution of 1,4-
bis(trimethylsilyl)-1,3-butadiyne (10 mmol) in anhydrous THF
(20 mL) at -78 °C and then stirred for 5 h at room temperature
under argon protection. Subsequently, 1-bromoalkane (12 mmol)
in fresh distilled HMPA (20 mL) was added dropwise at -78 °C
and stirred for 30 min at room temperature. After the pH was
adjusted to 7.0 by adding 3 M HCl at 0 °C, the mixture was
extracted with ethyl acetate. After the solvent was evaporated,
potassium fluoride dihydrate (20 mmol) in DMF (20 mL) was
added, stirred for 1 h at room temperature, poured into 3 M HCl
(15 mL) with stirring at 0 °C, and then extracted with ethyl
acetate. The organic layer was washed with 3 M HCl, brine,
saturated NaHCO3 solution, and saturated NaCl solution and
then dried over MgSO4. Removal of the solvent gave a brown
liquid, which was used for next step directly.
(d) Synthesis of Diacetylenic Alkylphosphonic Acids (Alkadiyn-
1-phosphonic Acids; 1a-1c in Scheme 1).39 Trimethylsilylbro-
mide (1.56 mmol) was added to an anhydrous CH2Cl2 solution
(5 mL) of diethyl alkadiyn-1-phosphonate (4, 0.26 mmol) at 0 °C
and then stirred overnight at room temperature. After the solvent
was evaporated under reduced pressure, water (15 mL) was added
and stirred for 24 h. The reaction mixture was extracted with ethyl
acetate, washed with water and saturated NaCl solution, and then
dried over MgSO4. Removal of the solvent gave a white solid. 4,6-
Nonadecadiyn-1-phosphonic acid (1a): Yield (from 4a) 79%. 1H
NMR (CDCl3) δ 2.26 (t, J = 7.2 Hz, 4H, C-CH2), 1.8-1.2 (m,
24H, CH2), 0.89 (t, J = 6.4 Hz, 3H, CH3). 13C NMR (CDCl3) δ
65.63, 65.39, 32.07, 29.64, 29.48, 29.30, 29.06, 28.54, 22.88, 19.40,
14.33. IR (cm-1): 2955, 2922, 2849, 1465, 1419, 1340, 1293, 1233,
1223, 1070, 1009, 989, 938, 724. Exact MS: m/z calculated for
C19H32O3P (M - H), 339.2089; found, 339.2080. 7,9-Nonadeca-
diyn-1-phosphonic acid (1b): Yield (from 4b) 82%. 1H NMR
(CDCl3) δ 2.23 (t, J = 6.8 Hz, 4H, C-CH2), 1.8-1.2 (m, 24H,
CH2), 0.89 (t, J = 6.4 Hz, 3H, CH3). 13C NMR (CDCl3) δ 65.54,
65.38, 32.10, 29.65, 29.49, 29.31, 29.25, 29.10, 29.03, 28.59, 22.89,
19.41, 14.34. IR (cm-1): 2955, 2921, 2849, 1465, 1420, 1340, 1293,
1232, 1223, 1065, 1009, 989, 938, 723. Exact MS: m/z calculated
for C19H34O3P (M - H), 339.2089; found, 339.2089. 10,12-Non-
adecadiyn-1-phosphonic acid (1c): Yield (from 4c) 79%. 1H NMR
(CDCl3) δ 2.24 (t, J = 6.4 Hz, 4H, C-CH2), 1.8-1.2 (m, 24H,
CH2), 0.88 (t, J = 6.4 Hz, 3H, CH3). 13C NMR (CDCl3) δ 65.60,
65.31, 32.07, 29.66, 29.49, 29.31, 29.22, 29.06, 29.00, 28.55, 22.65,
19.41, 14.35. IR (cm-1): 2954, 2920, 2848, 1464, 1420, 1342, 1293,
1233, 1222, 1072, 1011, 989, 935, 727. Exact MS: m/z calculated for
C19H32O3P (M þ H), 341.2246; found, 341.2242.
(c) Synthesis of Diethyl Alkadiyn-1-phosphonates (4a-4c in
Scheme 1). To crude 1,3-alkadiyne (3; nominally 2.0 mmol) in
anhydrous THF (8 mL) was added dropwise 1.5 mL of n-butyl-
lithium (a 1.6 M solution in hexane; 2.4 mmol) at 0 °C. The
reaction mixture was stirred for 1 h at 0 °C under argon, and then
a HMPA solution (8 mL) of diethyl (ω-bromoalkyl)phosphonate
Preparation and Characterization of Group-III Nitride
Samples. Group-III nitride samples coated with phosphonate
SAMs were prepared according to our previous report.18 In short,
GaN or AlGaN substrates (ca. 5 ꢀ 5 mm2) were soaked in 50%
HCl for 1 min, rinsed with water, dried under an Ar stream, and
then treated by UV/O3 for 30 min in a Novascan PSD-UVT UV/
(35) Eddy, C. R., Jr.; Holm, R. T.; Henry, R. L.; Culbertson, J. C.; Twigg, M. E.
J. Electron. Mater. 2005, 34, 1187–1192.
(36) Kosolapoff, G. M. J. Am. Chem. Soc. 1944, 66, 1511–1512.
(37) Xu, Z.; Byun, H.-S.; Bittman, R. J. Org. Chem. 1991, 56, 7183–7186.
(38) Kim, T.; Crooks, R. M. Tetrahedron Lett. 1994, 35, 9501–9504.
(39) Zamorano-Octaviano, J.; Hernandez-Martı
´
nez, A.; Ortega-Guevara, A.;
€
Linzaga-Elizalde, I.; Hopfl, H. Heteroat. Chem. 2006, 17, 75–80.
Langmuir 2010, 26(13), 10725–10730
DOI: 10.1021/la100273q 10727

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Li et al.
ozone system. The resulting UV/O3-treated substrates were im-
mersed in a 0.1 mM toluene solution of an organic compound for
17 h, thoroughly washed with toluene, and dried under an Ar
stream. Diacetylenic alkylphosphonate SAMs were polymerized
with a low intensity UV lamp (Spectronicsmodel EF-180, 254 nm,
1.9 mW cm-2 at 25 cm) at a distance of 10 cm in an Ar-purged
glovebag. Water contact angles on group-III nitride samples were
measured using a PG-1 pocket contact angle goniometer by
reading contact angle values of the two sides of a drop (2 μL)
within 30 s after deposition of the drop. To study the stability of
diacetylenic alkylphosphonate SAMs on group-III nitrides,
SAM-coated substrates were immersed in 0.1 M NaOH for
10-240 min at room temperature, quickly washed with water,
and dried under an Ar stream, and their water contact angles were
measured. Transmission visible absorption spectra of diacetylenic
alkylphosphonate SAMs on Al0.25Ga0.75N substrates deposited
onto double-side polished sapphire wafers were recorded on a
Varian Cary500 spectrophotometer. A UV/O3-treated substrate
was used as a reference, and diacetylenic alkylphosphonate SAMs
on the AlGaN after UV irradiation for 0, 2, 5, 6, 8, or 10 min were
used as samples. Contact mode AFM images were obtained by
contact-mode imaging in air, using a digital instruments multi-
mode atomic force microscope with Nanoscope IIIa electronics.
Contact-mode pyramidal AFM tips were purchased from Veeco
(model NP-10).
Table 1. Water Contact Angle (deg)a of Chemical Modified Group-III
Nitride Substratesb
GaN
Al0.2Ga0.8
N
Al0.25Ga0.75N
n-C18H37OH
n-C16H33SH
n-C18H37NH2
n-C17H35COOH
n-C18H37PO(OH)2
a Average ( standard deviation from three separate samples. b Sub-
strates treated in a UV/O3 cleaner for 30 min were immersed in a 0.1 mM
toluene solution for 17 h.
72.7 ( 1.2
71.7 ( 0.8
71.7 ( 1.4
75.0 ( 1.3
104.9 ( 1.2
63.3 ( 3.1
54.8 ( 1.3
63.1 ( 0.8
70.2 ( 0.7
103.1 ( 0.4
62.6 ( 1.6
56.7 ( 1.0
61.2 ( 2.2
67.2 ( 3.7
102.9 ( 0.1
and Ga2O3,40 whereas that on GaN consists only of Ga2O3.41-43
The lower adsorptivity (i.e., smaller θwater) of UV/O3-treated
AlGaN substrates may reflect the lower surface density of
hydrogen bonding sites on Al2O3 as compared to Ga2O3.44 The
stronger adsorption of acidic n-C17H35COOH on the AlGaN
substrates over basic n-C18H37NH2 is probably due to the higher
basicity and much lower acidity of Al2O3 surface.44,45 These θwater
values of the ODPA SAMs on GaN and AlGaN were smaller
than those previously reported on the Al2O3 surface of sputtered
Al.46 The larger θwater values in the literature may reflect the
surface roughness of the Al substrates and may also be due to the
use of a different method, the dynamic Wilhelmy method. Indeed,
our θwater value of the ODPA SAMs on single-crystal sapphire
substrateswas103°, close to our θwater valuesonGaN and AGaN.
Comparison of the Stability of Diacetylenic Alkylphos-
phonate SAMs on GaN in Basic Solution before and after
UV Irradiation. The above results suggest that ODPA SAMs
can be formed on UV/O3-treated GaN and AlGaN substrates. As
with ODPA (Table 1), the high θwater values of GaN and AlGaN
substrates (102-104°) were similarly observed after immersion in
a toluene solution containing one of diacetylenic alkylphosphonic
acids (1a-1c). The high θwater values suggest the SAM formation
of diacetylenic alkylphosphonic acids on GaN and AlGaN sub-
strates,18 permitting us to employ these substrates for investigat-
ing the properties of diacetylenic alkylphosphonate SAMs.
However, these SAMs without UV irradiation desorbed from
the substrates upon immersing in 0.1 M NaOH, as reported for
ODPA SAMs on GaN18,19 and Al2O3.46 For example, Figure 1
(open circles) shows the θwater values of 1b-coated GaN substrates
at different immersion times. Within 10 min, the θwater values
decreased from 103° to ca. 65° and then did not change upon
immersion for an extended time. In contrast, the desorption of 1b
SAMs on GaN substrates in 0.1 M NaOH was reduced by UV
irradiation at 254 nm (Figure 1, filled circles). The UV irradiation
led to a slight decrease in θwater from 103° to 96°, as shown by the
data at t = 0 min. The θwater of SAMs of 1a-1c on GaN and
AlGaN similarly decreased upon UV irradiation from 102-104°
to 95-96°, maybe due to changes in molecular packing and/or
partial degradation of the SAMs induced by the UV light.47 More
importantly, after the initial decrease from 96° within the first
10 min, the θwater values did not change to be ca. 82° upon prolonged
Results and Discussion
Water Contact Angle Measurements of GaN and AlGaN
Substrate Surfaces after the Adsorption of a Primary Sub-
stituted Hydrocarbon. Prior to investigating the properties of
diacetylenic alkylphosphonate SAMs, adsorption of primary sub-
stituted hydrocarbons with different terminal functional groups
(-OH, -SH, -NH2, -COOH, or -PO(OH)2) onto GaN and
AlGaN (x = 0.2, 0.25) was assessed by measuring the water
contact angle (θwater). We previously reported the essential con-
tribution of hydrogen bond interactions between polar functional
groups and surface oxide layers to the adsorption of these
compounds onto GaN substrates.17,18 Considering that the che-
mical properties (e.g., hydrogen bond acidity and basicity) of a
surface oxide layer would be altered according to the substrate
composition, the surfaces of group-III nitride substrates with
different Al/Ga ratios may exhibit different adsorptivities of these
compounds.
Just after UV/O3 treatment, the surfaces of AlGaN substrates
were hydrophilic (θwater < 15°), as with UV/O3-treated GaN
surfaces,17,18 suggesting the formation of a hydrophilic surface
oxide layer. After immersion in a 0.1 mM toluene solution for
17 h, which was long enough to reach the adsorption equilibrium,
the θwater values of AlGaN surfaces increased (Table 1) due to the
adsorption of a primary substituted hydrocarbon onto the sur-
faces. Regardless of the substrates, the adsorption of n-C18H37-
PO(OH)2 (ODPA) resulted in hydrophobic surfaces with very
high θwater values (>100°), suggesting ODPA SAM formation on
these substrates probably due to the strong hydrogen bond
acidity/basicity of the -PO(OH)2 group.18 In contrast, the other
four compounds did not strongly adsorb onto the substrates, as
shown by the much lower θwater values (<80°, Table 1). Inter-
estingly, the θwater values of AlGaN substrates upon the adsorp-
tion of these four compounds were significantly smaller than
those of GaN substrates. In addition, the difference in θwater
values between n-C17H35COOH and n-C18H37NH2 was larger on
AlGaN as compared to GaN.
(40) Hashizume, T.; Nakasaki, R.; Ootomo, S.; Oyama, S.; Hasegawa, H.
Mater. Sci. Eng., B 2001, 80, 309–312.
(41) Wolter, S. D.; Luther, B. P.; Waltemyer, D. L.; Onneby, C.; Mohney, S. E.;
Molnar, R. J. Appl. Phys. Lett. 1997, 70, 2156–2158.
(42) Shalish, I.; Shapira, Y.; Burstein, L.; Salzman, J. J. Appl. Phys. 2001, 89,
390–395.
(43) Shiozaki, N.; Sato, T.; Hashizume, T. Jpn. J. Appl. Phys. 2007, 46, 1471–
1473.
(44) Vimont, A.; Lavalley, J. C.; Sahibed-Dine, A.; Otero Arean, C.; Rodriguez
Delgado, M.; Daturi, M. J. Phys. Chem. B 2005, 109, 9656–9664.
(45) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371–6379.
(46) Liakos, I. L.; Newman, R. C.; McAlpine, E.; Alexander, M. R. Langmuir
2007, 23, 995–999.
The different molecular adsorption behavior between UV/O3-
treated GaN and AlGaN substrates may be explained on the basis
of the difference intheir surface chemical compositions. The oxide
layer on AlGaN surface would consist of the mixture of Al2O3
(47) Tanaka, T.; Honda, Y.; Sugi, M. Jpn. J. Appl. Phys. 1995, 34, 3250–3254.
10728 DOI: 10.1021/la100273q
Langmuir 2010, 26(13), 10725–10730

Li et al.
Article
Figure 2. Influence of UV irradiation time (tUV) on water contact
angle of diacetylenic phosphonate SAMs on GaN measured after
soaking in 0.1 M NaOH for 4 h at room temperature (θwater
1a; (b) 1b; (c) 1c. The plots and error bars indicate the average and
standard deviation, respectively, measured using three separate
samples.
NaOH): (a)
Figure 1. Water contact angles (θwater) of SAMs of diacetylenic
alkylphosphoric acid (1b) formed on GaN after soaking in 0.1 M
NaOH for 0, 10, 20, 30, 60, and 240 min at room temperature.
Filled and open circles represent data obtained for SAMs with and
without the UV irradiation (254 nm, 6 min), respectively. The plots
and error bars indicate the average and standard deviation, respec-
tively, measured using three separate samples.
immersion in 0.1 M NaOH. The significantly higher plateau θwater
values of UV-irradiated SAMs would indicate the improved che-
mical stability of the SAMs due to the photopolymerization of the
conjugated diacetylene moieties. On the other hand, the decrease in
θwater for the first 10 min of immersion may result from the partial
desorption of molecules at defect sites in the SAM (vide infra).
Effect of UV Irradiation Time on the Stability of Diace-
tylenic Alkylphosphonate SAMs on GaN in Basic Solution.
For all the diacetylenic alkylphosphonic acids examined, UV
irradiation led to the reduced desorption of their SAMs on GaN
in basic solution. Figure 2 summarizes the effect of UV irradiation
time (tUV) on the water contact angle of the SAMs of 1a-1c after
immersion in 0.1 M NaOH for 4 h (θwater
NaOH). Without UV
water
irradiation (i.e., at tUV = 0 min), θ
was ca. 60° for all the
NaOH
SAMs examined, indicating that these SAMs similarly desorbed
water
NaOH
in 0.1 M NaOH. With increasing tUV, θ
increased up to
ca. 80°, indicating the reduced desorption of the SAMs due to the
photopolymerization of conjugated diacetylene moieties in the
SAMs. However, prolonged UV irradiation led to a slight
decrease in θwater
NaOH, which would be due to the degradation of
the SAMs induced by the UV irradiation (vide infra).48,49 Inter-
estingly, tUV that gave the highest θwater
NaOH was longer for 1a, which
Figure 3. Visible absorption spectra of SAMs of 1c formed on
Al0.25Ga0.75N substrates at different UV irradiation times. A peak
around 580-600 nm originated from the substrate, which could
not be completely subtracted from the background visible absorp-
tion spectrum.
has a conjugated diacetylene moiety closer to a terminal phos-
phonic acid group, probably reflecting the feasibility of the
UV-initiated polymerization of diacetylene moieties. The photo-
polymerization and degradation of the SAMs could be verified
from their visible absorption spectra described in the next section.
Visible Absorption Spectra of Diacetylenic Alkylphos-
phonate SAMs on AlGaN. Figure 3 shows transmission visible
absorption spectra of a SAM of 1c on Al0.25Ga0.75N at different
tUV. At tUV = 0 min, except a peak around 580-600 nm due to
the AlGaN epilayer,50,51 there was no peak originating from the
diacetylenic alkylphosphonate SAM in the wavelength range of
500-700 nm. Upon UV irradiation, a broad peak around 640 nm
appeared. The height of the peak increased up to 6 min and then
decreased for longer tUV. The peak around 640 nm could be assi-
gned to π-π* electronic transition in the blue form of polydi-
acetylene.12,22,27,34 Indeed, the maximum absorbance obtained
at tUV = 6 min (ca. 0.002) was similar to that of a polydiacetylene
monolayer reported previously.32,47 On the other hand, a peak
around 540 nm due to the red form of polydiacetylene12,22,27,34
was not observed even for the prolonged tUV, indicating that the
decrease in 640 nm peak intensity for the longer tUV would be due
to the degradation of the polydiacetylene on AlGaN induced by
UV irradiation. The UV-induced degradation of polydiacetylene
was reported previously.48,49
Similar changes in the absorbance at 640 nm upon UV
irradiation were observed for SAMs of the diacetylenic alkylpho-
sphonic acids (1a-1c), as summarized in Figure 4. tUV that gave
the maximum absorbance (i.e., the maximum polymerization)
was longer in the order of 1a > 1b ∼ 1c. This order corresponded
to that of tUV required for maximum stability in basic solution
(Figure 2), supporting the claim that the improved stability of the
diacetylenic alkylphosphonate SAMs (Figure 1) was due to their
UV-initiated polymerization. In addition, the order suggests
photopolymerization was less efficient as the diacetylene moieties
were closer to the substrate (i.e., 1a), which was consistent to the
(48) Bloor, D.; Worboys, M. R. J. Mater. Chem. 1998, 8, 903–912.
(49) Morigaki, K.; Schonherr, H.; Frank, C. W.; Knoll, W. Langmuir 2003, 19,
6994–7002.
(50) Rieger, W.; Dimitrov, R.; Brunner, D.; Rohrer, E.; Ambacher, O.;
Stutzmann, M. Phys. Rev. B 1996, 54, 17596–17602.
(51) Mattila, T.; Nieminen, R. M. Phys. Rev. B 1997, 55, 9571–9576.
Langmuir 2010, 26(13), 10725–10730
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Article
Li et al.
Figure 5 shows topographic and friction images of a UV-irradiated
SAM of 1b on GaN before and after immersion in 0.1 M NaOH for
4 h. The topographic images prior to the immersion (Figure 5a, left)
showed step features that reflected the underlying surface structure
of a GaN substrate fabricated by the MOCVD method,9,17 suggest-
ing uniform coverage of the surface with the SAM. In contrast, after
immersion in 0.1 M NaOH, a number of pits were observed in the
topographic image, especially at the step edges (Figure 5b, left). The
surface heterogeneity was more clearly shown in the frictional
images (i.e., the right images of Figure 5a and b). The pits were
probably formed as a result of the desorption of the SAM molecules
that were not highly cross-linked at their defects. The preferential
desorption at the step edges may reflect the hindered topochemical
photopolymerization of diacetylene moieties at the step edges of the
substrate53 and also the higher accessibility of the basic solution to
the resulting defect sites in the SAM.
Figure 4. Relationship between the absorbance of diacetylenic
phosphonate SAMs formed on Al0.25Ga0.75N at 645 nm and UV
irradiation time (tUV): (a) 1a; (b) 1b; (c) 1c. The peak height was
determined by Gaussian fitting of a peak. The plots and error bars
indicate the average and standard deviation, respectively, mea-
sured using three separate samples.
Conclusions
This paper described the SAM formation of diacetylenic
alkylphosphonate SAMs on group-III nitride substrates and their
photopolymerization. UV/O3-treated AlGaN substrates could be
used to prepare alkylphosphonate SAMs, as with UV/O3-treated
GaN substrates. They exhibited weaker adsorption of the other
primary substituted hydrocarbons as compared to GaN maybe
due to the presence of Al2O3 having lower hydrogen bond acidity.
More importantly, the UV-initiated photopolymerization of
diacetylenic alkylphosphonate SAMs reduced their desorption in
aqueous basic solution. SAMs containing diacetylene moieties
farther from terminal phosphonate groups exhibited more effi-
cient photopolymerization. However, the hindrance of the topo-
chemical polymerization at the III-N surface step edges and the
UV-induced degradation of polydiacetylene on group-III nitride
surface limited the uniform functionalization with diacetylenic
alkylphosphonate SAMs. The results reported here will allow for
designing alkylphosphonic acid derivatives suitable to functiona-
lize group-III nitrides and other semiconductor surfaces14,54 not
only for future sensor applications but also for power sources
such as dye-sensitized solar cells.55
Figure 5. Contact mode topography (left) and friction (right)
images of a UV-irradiated (6 min) SAM of diacetylenic phosphoric
acid 1b on GaN (a) before and (b) after soaking in 0.1 M NaOH for
4 h at room temperature. The topography and friction images were
taken at the same area simultaneously.
trend observed in the photopolymerization yield of diacetyle-
nic thiolate SAMs on gold.52 The lower efficiency probably
reflected the hindrance of the topochemical polymerization due
to the limited flexibility of the diacetylene moieties.53 In
comparison, more efficient photopolymerization was observed
in a LB film of an amphiphilic compound having the same total
hydrophobic chain length with a diacetylene moiety near a
hydrophilic headgroup.23 In LB films, the head groups were
not completely fixed to the air/water interface, and thus the
photopolymerization efficiency was mainly determined by
molecular packing that was better for compounds with longer
alkyl chains.23
AFM Images of a Photopolymerized Diacetylenic Alkyl-
phosphonate SAM on GaN before and after Immersion in
Basic Solution. In addition, AFM images of a photopolyme-
rized diacetylenic SAM on GaN were measured to understand
its partial desorption in 0.1 M NaOH (Figure 1, filled circles).
Acknowledgment. The authors thank Dr. Duy H. Hua and
Dr. Huiping Zhao for their help with the organic synthesis, and
Dr. Kenneth J. Klabunde and Dr. Kennedy K. Kalebaila for their
assistance in the measurements of visible absorption spectra. They
gratefully acknowledge financial support from Targeted Excellence
Funds of Kansas State University. Work at the Naval Research
Laboratory is supported by the Office of Naval Research. J.K.H.
and E.S. acknowledge the supports of the American Society for
Engineering Education-Naval Research Laboratory Postdoctoral
Fellowship Program and the Kansas State University Developing
Scholars Program, respectively.
Supporting Information Available: 1H NMR, 13C NMR,
and IR spectra of compounds 1a-c, 2a-c, and 4a-c, and the
mass spectra of compounds 1a-c. This material is available
free of charge via the Internet at http://pubs.acs.org.
(52) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. J. Phys. Chem. B 1998,
102, 9550–9556.
(54) Mutin, P. H.; Guerrero, G.; Vioux, A. J. Mater. Chem. 2005, 15, 3761–3768.
(55) Wang, Y.; Yang, K.; Kim, S.-C.; Nagarajan, R.; Samuelson, L. A.; Kumar,
J. Chem. Mater. 2006, 18, 4215–4217.
(53) Mowery, M. D.; Menzel, H.; Cai, M.; Evans, C. E. Langmuir 1998, 14,
5594–5602.
10730 DOI: 10.1021/la100273q
Langmuir 2010, 26(13), 10725–10730